feat(imperative): support formatted tensor and add special op rules

GitOrigin-RevId: 77ff909f23

imperative/python/megengine/__init__.py

@@ -156,6 +156,7 @@ _atexit(_persistent_cache.flush)
 # subpackages
 import megengine.amp
 import megengine.autodiff
+import megengine.config
 import megengine.data
 import megengine.distributed
 import megengine.dtr

imperative/python/megengine/core/_config.py

@@ -2,7 +2,13 @@
 import os
 from contextlib import contextmanager
 
-from ._imperative_rt.core2 import _clear_algorithm_cache, get_option, set_option
+from ._imperative_rt.core2 import (
+    _clear_algorithm_cache,
+    get_auto_format_convert,
+    get_option,
+    set_auto_format_convert,
+    set_option,
+)
 
 __compute_mode = "default"
 __conv_format = "default"
@@ -24,8 +30,8 @@ __all__ = [
 def benchmark_kernel(mod):
     r"""Whether or not run possible algorithms on real device to find the best one. The default option is false,
     which means use heuristic to choose the fastest algorithm.
-    
-    Examples:    
+
+    Examples:
         .. code-block::
 
            import megengine as mge
@@ -47,8 +53,8 @@ def benchmark_kernel(mod, option: bool):
 def deterministic_kernel(mod):
     r"""Whether or not the fastest algorithm choosed is reproducible. The default option is false,
     which means the algorithm is not reproducible.
-    
-    Examples:    
+
+    Examples:
         .. code-block::
 
            import megengine as mge
@@ -67,8 +73,8 @@ def deterministic_kernel(mod, option: bool):
 def async_level(mod) -> int:
     r"""Get or set config whether raise error exactly when invoking op. The default level is 2,
     which means both device and user side errors are async.
-    
-    Examples:    
+
+    Examples:
         .. code-block::
 
            import megengine as mge
@@ -108,8 +114,8 @@ def _compute_mode(mod):
     which means that no special requirements will be placed on.  When set to 'float32', it
     would be used for accumulator and intermediate result, but only effective when input and 
     output are of float16 dtype.
-    
-    Examples:    
+
+    Examples:
         .. code-block::
 
            import megengine as mge
@@ -137,8 +143,8 @@ def _conv_format(mod):
     ``NCHW88`` layout: ``{N, C/8, H, W, 8}``
     ``CHWN4`` layout: ``{C/4, H, W, N, 4}``
     ``NCHW64`` layout: ``{N, C/64, H, W, 64}``
-    
-    Examples:    
+
+    Examples:
         .. code-block::
 
            import megengine as mge
@@ -153,20 +159,41 @@ def _conv_format(mod, format: str):
     __conv_format = format
 
 
+@property
+def _auto_format_convert(mod):
+    r"""Automatically convert indexing params' order for NCHW Tensor to NHWC order.
+    The default value is False, which means no convert.
+
+    Examples:
+        .. code-block::
+
+           import megengine as mge
+           mge.config._auto_format_convert = True
+    """
+    return get_auto_format_convert()
+
+
+@_auto_format_convert.setter
+def _auto_format_convert(mod, option: bool):
+    set_auto_format_convert(option)
+
+
 def _reset_execution_config(
     benchmark_kernel=None,
     deterministic_kernel=None,
     async_level=None,
     compute_mode=None,
     conv_format=None,
+    auto_format_convert=None,
 ):
-    global _benchmark_kernel, _deterministic_kernel, _async_level, __compute_mode, __conv_format
+    global _benchmark_kernel, _deterministic_kernel, __compute_mode, __conv_format
     orig_flags = (
         _benchmark_kernel,
         _deterministic_kernel,
         get_option("async_level"),
         __compute_mode,
         __conv_format,
+        get_auto_format_convert(),
     )
     if benchmark_kernel is not None:
         _benchmark_kernel = benchmark_kernel
@@ -178,6 +205,8 @@ def _reset_execution_config(
         __compute_mode = compute_mode
     if conv_format is not None:
         __conv_format = conv_format
+    if auto_format_convert is not None:
+        set_auto_format_convert(auto_format_convert)
 
     return orig_flags
 
@@ -189,26 +218,33 @@ def _override(
     async_level=None,
     compute_mode=None,
     conv_format=None,
+    auto_format_convert=None,
 ):
     r"""A context manager that users can opt in by attaching the decorator to set 
     the config of the global variable.
-    
-    Examples:    
+
+    Examples:
         .. code-block::
 
            import megengine as mge
-           
+
            @mge.config._override(
                 benchmark_kernel = True,
                 deterministic_kernel = Fasle,
                 async_level=2,
                 compute_mode="float32",
                 conv_format="NHWC",
+                auto_format_convert=True,
             )
            def train():
     """
     orig_flags = _reset_execution_config(
-        benchmark_kernel, deterministic_kernel, async_level, compute_mode, conv_format,
+        benchmark_kernel,
+        deterministic_kernel,
+        async_level,
+        compute_mode,
+        conv_format,
+        auto_format_convert,
     )
     try:
         yield

imperative/python/megengine/functional/vision.py

@@ -564,7 +564,6 @@ def interpolate(
         if inp.dtype == np.float16:
             inp = inp.astype("float32")
         conv_format = _config._get_actual_op_param("NCHW", _config.__conv_format)
-        assert conv_format == "NCHW", "Currently resize only support NCHW mode"
         op = builtin.Resize(imode=mode_map[mode], format=conv_format)
         shape = astensor1d(dsize, inp, dtype="int32", device=inp.device)
         (ret,) = apply(op, inp, shape)

imperative/python/megengine/tensor.py

@@ -4,6 +4,7 @@ from typing import Union
 import numpy as np
 
 from .core._imperative_rt import CompNode
+from .core._imperative_rt.core2 import FormatType
 from .core._imperative_rt.core2 import Tensor as _Tensor
 from .core._imperative_rt.core2 import apply, set_py_tensor_type
 from .core._trace_option import use_symbolic_shape
@@ -45,6 +46,8 @@ class Tensor(_Tensor, ArrayMethodMixin):
         is_const: Whether make it a ``ImutableTensor`` in tracing mode, refer to :class:`.jit.trace`.
         no_cache: Whether cache it for memory sharing.
         name: Used to improve convenience in graph operation on dumped model.
+        format: Used to indicate which memory format Tensor uses. It will not affect actual memory order or stride,
+            but may affect some operators related to indexing and dimension. Only support "default", "nchw" and "nhwc".
 
     .. note::
 
@@ -73,6 +76,7 @@ class Tensor(_Tensor, ArrayMethodMixin):
         is_const: bool = False,
         no_cache: bool = False,
         name: str = None,
+        format: str = "default",
     ):
         if name is None:
             name = ""
@@ -117,6 +121,10 @@ class Tensor(_Tensor, ArrayMethodMixin):
         return super().dtype
 
     @property
+    def format(self) -> str:
+        return super().format
+
+    @property
     def qparams(self):
         r"""Returns a :class:`~.QParams` object containing quantization params of a :class:`~.Tensor`."""
         from .quantization.utils import create_qparams  # pylint: disable=all

imperative/python/src/tensor.cpp

@@ -8,6 +8,7 @@
 #include "megbrain/imperative/transformations/dim_expansion.h"
 #include "megbrain/imperative/transformations/dtype_promote.h"
 #include "megbrain/imperative/transformations/eval.h"
+#include "megbrain/imperative/transformations/format.h"
 #include "megbrain/imperative/transformations/lazy.h"
 #include "megbrain/imperative/transformations/scalar.h"
 #include "megbrain/imperative/transformations/symbol.h"
@@ -492,6 +493,9 @@ ssize_t name2idx(const char* name) {
         // name
         case 'a': return compare_cstr<'m', 'e'>(ch) ? 5 : -1;
         }
+    case 'f':
+        // format
+        return compare_cstr<'o', 'r', 'm', 'a', 't'>(ch) ? 6 : -1;
     }
     // clang-format on
     return -1;
@@ -508,6 +512,7 @@ TensorWrapper::TensorWrapper(PyObject* args, PyObject* kwargs) {
                     {"is_const", []() -> py::object { return py::bool_(false); }},
                     {"no_cache", []() -> py::object { return py::bool_(false); }},
                     {"name", []() -> py::object { return py::none(); }},
+                    {"format", []() -> py::object { return py::none(); }},
             },
             name2idx};
     py::detail::loader_life_support life_sup;  // FIXME!!!required to cast DType
@@ -518,19 +523,23 @@ TensorWrapper::TensorWrapper(PyObject* args, PyObject* kwargs) {
     } else {
         tup = parse_args(tup, descs);
     }
-    mgb_assert(tup.size() == 6);
+    mgb_assert(tup.size() == 7);
     if (auto* t = try_cast(tup[0].ptr())) {
         m_tensor = t->m_tensor->copy();
     } else {
         auto data = tup[0];
         DType dtype = tup[1].cast<DType>();
+        CompNode cn = as_comp_node(tup[2]);
         bool is_const = tup[3].cast<bool>();
         bool no_cache = tup[4].cast<bool>();
         std::string name;
         if (!tup[5].is_none()) {
             name = tup[5].cast<std::string>();
         }
-        CompNode cn = as_comp_node(tup[2]);
+        Format format;
+        if (!tup[6].is_none()) {
+            format = tup[6].cast<std::string>();
+        }
 
         {
             CreateTensor::Kind kind = is_const ? CreateTensor::Const
@@ -544,7 +553,7 @@ TensorWrapper::TensorWrapper(PyObject* args, PyObject* kwargs) {
             } else {
                 auto&& hval = pyobj2hval(data, cn, dtype);
                 val = imperative::apply(
-                        CreateTensor(kind, cn, hval.dtype, hval.shape),
+                        CreateTensor(kind, cn, hval.dtype, hval.shape, format),
                         hval.storage)[0];
             }
             m_tensor.emplace(val);
@@ -610,6 +619,10 @@ PyObject* TensorWrapper::device() {
     return py::cast(m_tensor->comp_node()).release().ptr();
 }
 
+PyObject* TensorWrapper::format() {
+    return py::cast(m_tensor->format().to_string()).release().ptr();
+}
+
 PyObject* TensorWrapper::numpy() {
     auto hv = m_tensor->numpy();
     if (!hv) {
@@ -722,6 +735,7 @@ WRAP_FUNC_PY35(pixel_shuffle_cpp);
 void init_tensor(py::module m) {
     imperative::Tensor::static_initialize();
 
+    // Transformations
     static auto& transformations = TransformationManager::get_instance();
 
     using Segment = TransformationManager::Segment;
@@ -755,6 +769,9 @@ void init_tensor(py::module m) {
                               .register_at<Segment::DimExpansion>(
                                       std::make_shared<DimExpansionTransformation>())
                               .release());
+    auto format_trans = std::make_shared<FormatTransformation>();
+    MGB_MARK_USED_VAR(
+            transformations.register_at<Segment::Format>(format_trans).release());
 
     static py::exception<interpreter::AsyncError> py_async_error(
             m, "AsyncError", PyExc_RuntimeError);
@@ -788,12 +805,14 @@ void init_tensor(py::module m) {
         }
     });
 
+    // Tensor
     auto* tensor_type =
             TensorWrapper::wrap_t::type()
                     .def<&TensorWrapper::numpy>("numpy")
                     .def_getset<&TensorWrapper::shape>("shape")
                     .def_getset<&TensorWrapper::dtype>("dtype")
                     .def_getset<&TensorWrapper::device>("device")
+                    .def_getset<&TensorWrapper::format>("format")
                     .def<&TensorWrapper::reset>("_reset")
                     .def<&TensorWrapper::isscalar>("_isscalar")
                     .def<&TensorWrapper::detach>("detach")
@@ -812,6 +831,11 @@ void init_tensor(py::module m) {
     if (!tensor_type)
         throw py::error_already_set();
     py::setattr(m, "Tensor", tensor_type);
+    py::enum_<Format::Type>(m, "FormatType")
+            .value("DEFAULT", Format::Type::DEFAULT)
+            .value("NCHW", Format::Type::NCHW)
+            .value("NHWC", Format::Type::NHWC)
+            .export_values();
 
     py::class_<TensorWeakRef>(m, "TensorWeakRef")
             .def(py::init<const TensorWrapper&>())
@@ -911,6 +935,7 @@ void init_tensor(py::module m) {
         sync_py_task_q();
     });
 
+    // GradTransformation
     py::handle grad_key_type =
             GradKeyWrapper::wrap_t::type()
                     .def<&GradKeyWrapper::attach>("attach")
@@ -1203,6 +1228,7 @@ void init_tensor(py::module m) {
         return wrapped_outputs;
     });
 
+    // ModuleTraceTransformation
     static py::function module_trace_hook;
 
     static auto get_module_trace = [] {
@@ -1309,6 +1335,12 @@ void init_tensor(py::module m) {
 
     m.def("_clear_algorithm_cache", [] { megdnn::AlgorithmCache::instance().clear(); });
 
+    // FormatTransformation
+    m.def("set_auto_format_convert",
+          [format_trans](bool enabled) { format_trans->set_auto_convert(enabled); });
+    m.def("get_auto_format_convert",
+          [format_trans]() { return format_trans->get_auto_convert(); });
+
     py::register_exception<TraceError>(m, "TraceError");
 }
 

imperative/python/src/tensor.h

@@ -1,10 +1,11 @@
 #pragma once
 #pragma GCC diagnostic ignored "-Wmissing-field-initializers"
 
-#include <variant>
-
 #include <string>
 #include <unordered_map>
+#include <variant>
+
+#include "megbrain/imperative/dispatch.h"
 #include "megbrain/imperative/interpreter.h"
 #include "pybind11/pybind11.h"
 
@@ -57,6 +58,7 @@ public:
         }
         return *shape;
     }
+    inline Format format() { return *data().format(); }
     inline HostValue::ref_t numpy() { return data().numpy(); }
     inline void reset(ValueRef value) {
         m_data = value;
@@ -116,6 +118,7 @@ public:
     PyObject* shape();
     PyObject* dtype();
     PyObject* device();
+    PyObject* format();
     PyObject* numpy();
     void reset(PyObject*);
     PyObject* detach();

imperative/python/src/transformation.h

@@ -19,6 +19,7 @@ public:
         DTypePromote,
         DimExpansion,
         Grad,
+        Format,
         Scalar,
         Symbol,
         Trace,

imperative/python/test/unit/amp/test_autocast.py

@@ -2,7 +2,7 @@ from megengine import amp
 from megengine.core.tensor import amp as origin_amp
 
 
-def test_grad_scaler():
+def test_autocast():
     def check(enabled, low, high):
         assert amp.enabled == enabled
         assert origin_amp._enabled == enabled

imperative/python/test/unit/core/test_formatted_tensor.py

@@ -0,0 +1,307 @@
+import numpy as np
+import pytest
+
+import megengine as mge
+import megengine.functional as F
+from megengine import tensor
+from megengine.autodiff import GradManager
+
+
+def test_basic():
+    a = tensor(np.arange(0, 24).reshape((1, 2, 3, 4)), dtype="float32", format="nhwc")
+    assert a.format == "nhwc"
+    b = tensor(a)
+    assert b.format == "nhwc"
+    # TODO: fix Tensor init bug for another Tensor
+    # c = tensor(a, format="nchw")
+    # assert c.format == "nchw"
+
+
+def _compare_nchw_nhwc(data, func):
+    x1 = tensor(data, format="nchw")
+    x2 = tensor(data.transpose(0, 2, 3, 1), format="nhwc")
+    out1 = func(x1)
+    with mge.config._override(auto_format_convert=True):
+        out2 = func(x2)
+    np.testing.assert_equal(out1, out2)
+
+
+def test_dimshuffle():
+    def func(x):
+        out = F.transpose(x, [2, 3, 0, 1])
+        assert out.format == "default"
+        return out.numpy()
+
+    data = np.arange(0, 24).reshape((1, 2, 3, 4))
+    _compare_nchw_nhwc(data, func)
+
+
+def test_reshape():
+    # maintain NHWC format
+    def func(x):
+        out = F.reshape(x, (1, 2, 6, 2))
+        if x.format == "nhwc":
+            assert out.format == "nhwc"
+        return out.numpy()
+
+    data = np.arange(0, 24).reshape((1, 2, 3, 4))
+    _compare_nchw_nhwc(data, func)
+
+    # not maintain NHWC format
+    def func2(x):
+        out = F.reshape(x, (1, 24))
+        assert out.format == "default"
+        return out.numpy()
+
+    _compare_nchw_nhwc(data, func2)
+
+
+def test_flatten():
+    def func(x):
+        return F.flatten(x).numpy()
+
+    data = np.arange(0, 24).reshape((1, 2, 3, 4))
+    _compare_nchw_nhwc(data, func)
+
+
+def test_broadcast():
+    # maintain NHWC format
+    def func(x):
+        out = F.broadcast_to(x, (4, 3, 2, 3))
+        if x.format == "nhwc":
+            assert out.format == "nhwc"
+        return out.numpy()
+
+    data = np.arange(0, 24).reshape((4, 3, 2, 1))
+    _compare_nchw_nhwc(data, func)
+
+    # not maintain NHWC format
+    def func2(x):
+        out = F.broadcast_to(x, (3, 4, 3, 2, 1))
+        assert out.format == "default"
+        return out.numpy()
+
+    _compare_nchw_nhwc(data, func2)
+
+
+@pytest.mark.skip("repeat cannot maintain format yet")
+def test_repeat():
+    def func(x):
+        rst = F.repeat(x, 3, axis=1)
+        assert rst.format == x.format
+        return rst.numpy()
+
+    data = np.arange(0, 24).reshape((1, 2, 3, 4))
+    _compare_nchw_nhwc(data, func)
+
+
+def test_getshape():
+    def func(x):
+        return x.shape
+
+    data = np.arange(0, 24).reshape((1, 2, 3, 4))
+    _compare_nchw_nhwc(data, func)
+
+
+@pytest.mark.skip("symbolic shape is not supported yet")
+def test_get_symbolic_shape():
+    from megengine.core._trace_option import set_symbolic_shape
+
+    origin_opt = set_symbolic_shape(True)
+
+    def func(x):
+        return x.shape.numpy()
+
+    data = np.arange(0, 24).reshape((1, 2, 3, 4))
+    _compare_nchw_nhwc(data, func)
+    set_symbolic_shape(origin_opt)
+
+
+def test_getvalue():
+    def func(x):
+        return x.numpy()
+
+    data = np.arange(0, 24).reshape((1, 2, 3, 4))
+    _compare_nchw_nhwc(data, func)
+
+
+def test_get_set_subtensor():
+    def get_subtensor(x):
+        return x[:, :1, :2, :3].numpy()
+
+    data = np.arange(0, 24).reshape((1, 2, 3, 4))
+    _compare_nchw_nhwc(data, get_subtensor)
+
+    def set_subtensor(x):
+        x[:, :1, :2, :3] = 0
+        return x.numpy()
+
+    _compare_nchw_nhwc(data, set_subtensor)
+
+
+def test_get_set_advanced_indexing():
+    def get_advanced_indexing(x):
+        x = x[:, : mge.tensor(2), : mge.tensor(2), [1, 2]].numpy()
+        return x
+
+    data = np.arange(0, 24).reshape((1, 2, 3, 4))
+    _compare_nchw_nhwc(data, get_advanced_indexing)
+
+    def set_advanced_indexing(x):
+        x[:, : mge.tensor(2), : mge.tensor([2]), [1,]] = 0
+        return x.numpy()
+
+    _compare_nchw_nhwc(data, set_advanced_indexing)
+
+
+def test_typecvt():
+    def typecvt(x):
+        return x.astype("float16").numpy()
+
+    data = np.arange(0, 24).reshape((1, 2, 3, 4))
+    _compare_nchw_nhwc(data, typecvt)
+
+
+def test_elemwise():
+    def elemwise(x):
+        return (x * 2 + x / 2).numpy()
+
+    data = np.arange(0, 24).reshape((1, 2, 3, 4))
+    _compare_nchw_nhwc(data, elemwise)
+
+
+def test_concat():
+    def func(x):
+        rst = F.concat([x / 2, x * 2], axis=1)
+        assert rst.format == x.format
+        return rst.numpy()
+
+    data = np.arange(0, 24).reshape((1, 2, 3, 4))
+    _compare_nchw_nhwc(data, func)
+
+
+@pytest.mark.parametrize(
+    "mode", ["bilinear", "nearest"],
+)
+def test_interpolate(mode):
+    def func(x):
+        if x.format == "nhwc":
+            with mge.config._override(conv_format="NHWC"):
+                rst = F.vision.interpolate(x, scale_factor=3, mode=mode)
+                assert rst.format == "nhwc"
+                return rst.numpy()
+        else:
+            return F.vision.interpolate(x, scale_factor=3, mode=mode).numpy()
+
+    # NHWC interpolate only suppoted channel is 1 or 3
+    data = np.arange(0, 48).reshape((1, 3, 4, 4)).astype("float32")
+    _compare_nchw_nhwc(data, func)
+
+
+def test_conv2d():
+    def conv2d(x):
+        if x.format == "nhwc":
+            with mge.config._override(conv_format="NHWC"):
+                x = F.conv2d(
+                    x,
+                    weight=mge.tensor(np.ones((3, 1, 1, 2)), format="nhwc"),
+                    bias=mge.tensor(np.ones((1, 1, 1, 3)), format="nhwc"),
+                )
+                assert x.format == "nhwc"
+                return x.numpy()
+        else:
+            return F.conv2d(x, F.ones((3, 2, 1, 1)), F.ones((1, 3, 1, 1))).numpy()
+
+    data = np.arange(0, 24).reshape((1, 2, 3, 4))
+    _compare_nchw_nhwc(data, conv2d)
+
+
+def test_group_conv2d():
+    def conv2d(x):
+        if x.format == "nhwc":
+            with mge.config._override(conv_format="NHWC"):
+                x = F.conv2d(
+                    x,
+                    weight=mge.tensor(np.ones((2, 2, 1, 1, 2)), format="nhwc"),
+                    bias=mge.tensor(np.ones((1, 1, 1, 4)), format="nhwc"),
+                    groups=2,
+                )
+                assert x.format == "nhwc"
+                return x.numpy()
+        else:
+            return F.conv2d(
+                x, F.ones((2, 2, 2, 1, 1)), F.ones((1, 4, 1, 1)), groups=2
+            ).numpy()
+
+    data = np.arange(0, 48).reshape((1, 4, 3, 4))
+    _compare_nchw_nhwc(data, conv2d)
+
+
+def test_bn():
+    def func(x):
+        if x.format == "nhwc":
+            with mge.config._override(bn_format="dim_111c"):
+                oups = F.batch_norm(
+                    x.astype("float32"),
+                    running_mean=mge.tensor(np.ones((1, 1, 1, 2)), format="nhwc"),
+                    running_var=mge.tensor(np.ones((1, 1, 1, 2)), format="nhwc"),
+                    weight=mge.tensor(np.ones((1, 1, 1, 2)), format="nhwc"),
+                    bias=mge.tensor(np.ones((1, 1, 1, 2)), format="nhwc"),
+                    training=True,
+                    inplace=False,
+                )
+                assert oups[0].format == "nhwc", "y's format is wrong"
+                assert oups[1].format == "nhwc", "running_mean's format is wrong"
+                assert oups[2].format == "nhwc", "running_var's format is wrong"
+                return oups[0].numpy()
+        else:
+            return F.batch_norm(
+                x.astype("float32"),
+                running_mean=mge.tensor(np.ones((1, 2, 1, 1))),
+                running_var=mge.tensor(np.ones((1, 2, 1, 1))),
+                weight=mge.tensor(np.ones((1, 2, 1, 1))),
+                bias=mge.tensor(np.ones((1, 2, 1, 1))),
+                training=True,
+                inplace=False,
+            )[0].numpy()
+
+    data = np.arange(0, 24).reshape((1, 2, 3, 4))
+    _compare_nchw_nhwc(data, func)
+
+
+@pytest.mark.parametrize(
+    "pooling",
+    [F.max_pool2d, F.avg_pool2d, F.adaptive_avg_pool2d, F.adaptive_max_pool2d],
+)
+def test_pooling2d(pooling):
+    def func(x):
+        if x.format == "nhwc":
+            with mge.config._override(conv_format="NHWC"):
+                x = pooling(x.astype("float32"), 2)
+                assert x.format == "nhwc"
+                return x.numpy()
+        else:
+            return pooling(x.astype("float32"), 2).numpy()
+
+    data = np.arange(0, 24).reshape((1, 2, 3, 4))
+    _compare_nchw_nhwc(data, func)
+
+
+def test_backward():
+    data = np.arange(0, 24).reshape((1, 2, 3, 4))
+    x = tensor(data.transpose(0, 2, 3, 1), format="nhwc")
+    w = mge.tensor(np.ones((3, 1, 1, 2)), format="nhwc")
+    b = mge.tensor(np.ones((1, 1, 1, 3)), format="nhwc")
+    gm = GradManager().attach([w, b])
+    with gm:
+        with mge.config._override(auto_format_convert=True, conv_format="NHWC"):
+            x = F.conv2d(x, w, b)
+            gm.backward(x)
+            # TODO: backward grad has no format yet
+            np.testing.assert_equal(
+                w.grad.numpy(),
+                np.array([66, 210, 66, 210, 66, 210]).reshape((3, 1, 1, 2)),
+            )
+            np.testing.assert_equal(
+                b.grad.numpy(), np.array([12, 12, 12]).reshape((1, 1, 1, 3))
+            )

imperative/src/impl/basic_operators.cpp

@@ -33,14 +33,20 @@ std::string GetAttr::to_string() const {
     return ssprintf("GetAttr{attr=%s}", attr_name);
 }
 
-CreateTensor::CreateTensor(Kind kind, CompNode device, DType dtype, ValueShape shape)
-        : m_kind(kind), m_device(device), m_dtype(dtype), m_shape(shape) {}
+CreateTensor::CreateTensor(
+        Kind kind, CompNode device, DType dtype, ValueShape shape, Format format)
+        : m_kind(kind),
+          m_device(device),
+          m_dtype(dtype),
+          m_shape(shape),
+          m_format(format) {}
 
 CreateTensor::CreateTensor(Kind kind, CompNode device, TensorLayout layout)
         : m_kind(kind),
           m_device(device),
           m_dtype(layout.dtype),
-          m_shape(ValueShape::from(layout)) {
+          m_shape(ValueShape::from(layout)),
+          m_format(Format::Type::DEFAULT) {
     mgb_assert(
             layout.is_contiguous() || layout.is_empty(), "layout should be contiguous");
 }
@@ -74,8 +80,9 @@ auto CreateTensor::parse(Span<ValueRef> inputs) const -> Args {
 
 std::string CreateTensor::to_string() const {
     return ssprintf(
-            "CreateTensor{kind=%d, device=%s, dtype=%s, shape=%s}", (int)m_kind,
-            m_device.to_string().c_str(), m_dtype.name(), m_shape.to_string().c_str());
+            "CreateTensor{kind=%d, device=%s, dtype=%s, shape=%s, format=%s}",
+            (int)m_kind, m_device.to_string().c_str(), m_dtype.name(),
+            m_shape.to_string().c_str(), m_format.to_string().c_str());
 }
 
 std::string DTRCommand::to_string() const {

imperative/src/impl/transformations/format.cpp

@@ -0,0 +1,406 @@
+#include "megbrain/imperative/transformations/format.h"
+
+#include "megbrain/imperative/ops/autogen.h"
+
+namespace mgb {
+namespace imperative {
+
+using FT = Format::Type;
+
+TypedValueRef<FormattedTensorValue> FormattedTensorValue::as(const FT& target) const {
+    return FormattedTensorValue::make(m_value, target);
+}
+
+TypedValueRef<FormattedTensorValue> FormattedTensorValue::to(
+        const FT& target, const std::string& scope) const {
+    std::vector<int32_t> pattern;
+    if (m_format == FT::NHWC && target == FT::NCHW) {
+        pattern = {0, 3, 1, 2};
+    } else if (m_format == FT::NCHW && target == FT::NHWC) {
+        pattern = {0, 2, 3, 1};
+    } else {
+        mgb_throw(
+                MegBrainError, "Unsupport format conversion from %s to %s",
+                m_format.to_string().c_str(), Format(target).to_string().c_str());
+    }
+    auto output = imperative::apply(
+            *Dimshuffle::make(pattern, scope), std::vector<ValueRef>{m_value})[0];
+    return FormattedTensorValue::make(output, target);
+}
+
+namespace {
+
+ValueRef unwrap_input(const ValueRef& input) {
+    if (auto format_input = input.as_ref<FormattedTensorValue>()) {
+        return format_input->value();
+    } else {
+        return input;
+    }
+}
+
+std::vector<ValueRef> unwrap_inputs(const Span<ValueRef>& inputs) {
+    std::vector<ValueRef> unwrapped_inputs;
+    for (auto&& input : inputs) {
+        unwrapped_inputs.push_back(unwrap_input(input));
+    }
+    return unwrapped_inputs;
+}
+
+std::vector<ValueRef> wrap_outputs(
+        const std::vector<ValueRef>& outputs, FT type = FT::DEFAULT) {
+    std::vector<ValueRef> wrapped_outputs;
+    for (auto&& output : outputs) {
+        wrapped_outputs.push_back(FormattedTensorValue::make(output, type));
+    }
+    return wrapped_outputs;
+}
+
+ValueShape convert_nhwc2nchw_shape(const ValueShape& shape) {
+    mgb_assert(shape.ndim == 4);
+    auto out = ValueShape(shape);
+    out[3] = shape[2];
+    out[2] = shape[1];
+    out[1] = shape[3];
+    return out;
+}
+
+using FormatRule = std::function<std::vector<ValueRef>(
+        const OpDef&, Span<ValueRef>&, const bool&)>;
+static std::unordered_map<Typeinfo*, FormatRule> format_rules;
+
+template <typename T>
+void register_format_rule(
+        std::vector<ValueRef> (*rule)(const T&, Span<ValueRef>&, const bool&)) {
+    format_rules[T::typeinfo()] = [rule](const OpDef& def, Span<ValueRef>& inputs,
+                                         const bool& auto_convert) {
+        return (*rule)(def.cast_final_safe<T>(), inputs, auto_convert);
+    };
+}
+
+auto convert_nchw2nhwc_pattern(const std::vector<int32_t>& pattern) {
+    mgb_assert(pattern.size() == 4);
+    auto nhwc_pattern = pattern;
+    for (size_t idx = 0; idx < 4; ++idx) {
+        auto dim = pattern[idx];
+        if (dim == 1) {
+            nhwc_pattern[idx] = 3;
+        } else if (dim == 2) {
+            nhwc_pattern[idx] = 1;
+        } else if (dim == 3) {
+            nhwc_pattern[idx] = 2;
+        }
+    }
+    return nhwc_pattern;
+}
+
+std::vector<ValueRef> dimshuffle_rule(
+        const Dimshuffle& op, Span<ValueRef>& inputs, const bool& auto_convert) {
+    mgb_assert(inputs.size() == 1);
+    auto& src = inputs[0].cast<FormattedTensorValue>();
+    // Only support converting pattern from NCHW to NHWC currently.
+    if (auto_convert && src.format() == FT::NHWC) {
+        auto pattern = convert_nchw2nhwc_pattern(op.pattern);
+        // dimshuffle will not maintain NHWC Format
+        return wrap_outputs(imperative::apply(
+                *Dimshuffle::make(std::move(pattern), op.scope()),
+                unwrap_inputs(inputs)));
+    }
+    return wrap_outputs(imperative::apply(op, unwrap_inputs(inputs)));
+}
+
+ValueRef convert_nchw2nhwc_tensornd(const HostTensorND& shape) {
+    mgb_assert(shape.layout().total_nr_elems() == 4);
+    auto* old_ptr = shape.ptr<dt_int32>();
+    auto cn = shape.comp_node();
+    auto layout = shape.layout();
+    auto nhwc_shape = HostTensorND(cn, layout);
+    auto* new_ptr = nhwc_shape.ptr<dt_int32>();
+    new_ptr[0] = old_ptr[0];
+    new_ptr[1] = old_ptr[2];
+    new_ptr[2] = old_ptr[3];
+    new_ptr[3] = old_ptr[1];
+    auto hv = HostStorage::make(nhwc_shape.storage());
+    auto nhwc_shape_input =
+            imperative::apply(CreateTensor(CreateTensor::Const, cn, layout), hv)[0];
+    return nhwc_shape_input;
+}
+
+std::vector<ValueRef> reshape_rule(
+        const Reshape& op, Span<ValueRef>& inputs, const bool& auto_convert) {
+    mgb_assert(inputs.size() == 2);
+    auto& src = inputs[0].cast<FormattedTensorValue>();
+    if (auto_convert && src.format() == FT::NHWC) {
+        auto shape = unwrap_input(inputs[1]).numpy().cast<HostValue>().as_nd();
+        if (shape.layout().total_nr_elems() == 4) {
+            // output is still NHWC format
+            auto nhwc_shape = convert_nchw2nhwc_tensornd(shape);
+            auto outputs = imperative::apply(
+                    op, std::vector<ValueRef>{unwrap_input(inputs[0]), nhwc_shape});
+            return wrap_outputs(outputs, FT::NHWC);
+        } else {
+            // will not maintain src's format
+            auto nchw_src = src.to(FT::NCHW, op.scope())->value();
+            auto outputs = imperative::apply(
+                    op, std::vector<ValueRef>{nchw_src, unwrap_input(inputs[1])});
+            return wrap_outputs(outputs);
+        }
+    }
+    return wrap_outputs(imperative::apply(op, unwrap_inputs(inputs)));
+}
+
+std::vector<ValueRef> broadcast_rule(
+        const Broadcast& op, Span<ValueRef>& inputs, const bool& auto_convert) {
+    mgb_assert(inputs.size() == 2);
+    auto& src = inputs[0].cast<FormattedTensorValue>();
+    if (auto_convert && src.format() == FT::NHWC) {
+        auto shape = unwrap_input(inputs[1]).numpy().cast<HostValue>().as_nd();
+        if (shape.layout().total_nr_elems() == 4) {
+            // output is still NHWC format
+            auto nhwc_shape = convert_nchw2nhwc_tensornd(shape);
+            auto outputs = imperative::apply(
+                    op, std::vector<ValueRef>{unwrap_input(inputs[0]), nhwc_shape});
+            return wrap_outputs(outputs, FT::NHWC);
+        } else {
+            // will not maintain src's format
+            auto nchw_src = src.to(FT::NCHW, op.scope())->value();
+            auto outputs = imperative::apply(
+                    op, std::vector<ValueRef>{nchw_src, unwrap_input(inputs[1])});
+            return wrap_outputs(outputs);
+        }
+    }
+    return wrap_outputs(imperative::apply(op, unwrap_inputs(inputs)));
+}
+
+bool is_reduce_ndim_idx_items(
+        const std::vector<std::tuple<int8_t, bool, bool, bool, bool>>& items,
+        const Span<ValueRef>& inputs) {
+    for (auto i = 0; i < items.size(); ++i) {
+        auto&& [axis, begin, end, step, idx] = items[i];
+        if (idx) {
+            // if inputs[i] contains more than one value, ndim will not be reduced.
+            return inputs[i].is_scalar();
+        }
+    }
+    return false;
+}
+
+auto convert_nchw2nhwc_idx_items(
+        const std::vector<std::tuple<int8_t, bool, bool, bool, bool>>& items) {
+    auto nhwc_items = items;
+    for (auto i = 0; i < nhwc_items.size(); ++i) {
+        auto&& [axis, begin, end, step, idx] = nhwc_items[i];
+        if (axis == 2 || axis == 3) {
+            nhwc_items[i] = {axis - 1, begin, end, step, idx};
+        } else if (axis == 1) {
+            nhwc_items[i] = {3, begin, end, step, idx};
+        }
+    }
+    return nhwc_items;
+}
+
+template <typename T>
+std::vector<ValueRef> subtensor_rule(
+        const T& op, Span<ValueRef>& inputs, const bool& auto_convert) {
+    mgb_assert(inputs.size() >= 1);
+    auto& src = inputs[0].cast<FormattedTensorValue>();
+    bool is_reduce_ndim = is_reduce_ndim_idx_items(
+            op.items, {&inputs[1], &inputs[inputs.size() - 1]});
+    if (!is_reduce_ndim) {
+        // only support NHWC2NCHW convert, otherwise maintain src's format
+        if (!(auto_convert && src.format() == FT::NHWC)) {
+            return {FormattedTensorValue::make(
+                    imperative::apply(op, unwrap_inputs(inputs))[0], src.format())};
+        }
+        auto nhwc_items = convert_nchw2nhwc_idx_items(op.items);
+        auto outputs = imperative::apply(
+                *T::make(std::move(nhwc_items), op.scope()), unwrap_inputs(inputs));
+        return wrap_outputs(outputs, FT::NHWC);
+    }
+    return wrap_outputs(imperative::apply(op, unwrap_inputs(inputs)));
+}
+
+template <typename T>
+std::vector<ValueRef> setsubtensor_rule(
+        const T& op, Span<ValueRef>& inputs, const bool& auto_convert) {
+    mgb_assert(inputs.size() >= 2);
+    auto& src = inputs[0].cast<FormattedTensorValue>();
+    bool is_reduce_ndim = is_reduce_ndim_idx_items(
+            op.items, {&inputs[2], &inputs[inputs.size() - 1]});
+    if (!is_reduce_ndim) {
+        // only support NHWC2NCHW convert, otherwise maintain src's format
+        if (!(auto_convert && src.format() == FT::NHWC)) {
+            return {FormattedTensorValue::make(
+                    imperative::apply(op, unwrap_inputs(inputs))[0], src.format())};
+        }
+        // value has been broadcasted to src's fake NCHW shape.
+        auto& value = inputs[1].cast<FormattedTensorValue>();
+        auto& format = value.format();
+        auto nhwc_inputs = std::vector<ValueRef>(inputs.size());
+        if (format == FT::DEFAULT || format == FT::NCHW) {
+            // value for setsubtensor should transpose to match shape.
+            auto nhwc_value = value.as(FT::NCHW)->to(FT::NHWC);
+            // make new inputs for setsubtensor
+            nhwc_inputs[0] = src.value();
+            nhwc_inputs[1] = nhwc_value->value();
+            for (auto i = 2; i < inputs.size(); ++i) {
+                nhwc_inputs[i] = inputs[i].as_ref<FormattedTensorValue>()->value();
+            }
+        } else if (format != FT::NHWC) {
+            mgb_throw(
+                    MegBrainError, "Unsupported format(%s) of value for setsubtensor.",
+                    format.to_string().c_str());
+        }
+        auto nhwc_items = convert_nchw2nhwc_idx_items(op.items);
+        auto outputs = imperative::apply(
+                *T::make(std::move(nhwc_items), op.scope()), nhwc_inputs);
+        return wrap_outputs(outputs, FT::NHWC);
+    }
+    return wrap_outputs(imperative::apply(op, unwrap_inputs(inputs)));
+}
+
+FT get_inputs_format(Span<ValueRef>& inputs) {
+    FT format(FT::DEFAULT);
+    for (auto& inp : inputs) {
+        auto& inp_format = inp.cast<FormattedTensorValue>().format();
+        if (inp_format != FT::DEFAULT) {
+            mgb_assert(format == FT::DEFAULT || inp_format == format);
+            format = inp_format.type();
+        }
+    }
+    return format;
+}
+
+std::vector<ValueRef> concat_rule(
+        const Concat& op, Span<ValueRef>& inputs, const bool& auto_convert) {
+    FT format = get_inputs_format(inputs);
+    if (!(format == FT::NHWC && auto_convert)) {
+        return wrap_outputs(imperative::apply(op, unwrap_inputs(inputs)), format);
+    }
+    // TODO: handle 5D NHWC Tensor from group conv
+    auto axis = op.axis;
+    if (axis == 2 || axis == 3) {
+        axis = axis - 1;
+    } else if (axis == 1) {
+        axis = 3;
+    }
+    return wrap_outputs(
+            imperative::apply(
+                    *Concat::make(axis, op.comp_node, op.scope()),
+                    unwrap_inputs(inputs)),
+            format);
+}
+
+std::vector<ValueRef> elemwise_rule(
+        const Elemwise& op, Span<ValueRef>& inputs, const bool& auto_convert) {
+    FT format = get_inputs_format(inputs);
+    return wrap_outputs(imperative::apply(op, unwrap_inputs(inputs)), format);
+}
+
+std::vector<ValueRef> identity_rule_helper(
+        const OpDef& op, const Span<ValueRef>& inputs) {
+    // mgb_assert(inputs.size() == 1);
+    auto& src = inputs[0].cast<FormattedTensorValue>();
+    return wrap_outputs(
+            imperative::apply(op, unwrap_inputs(inputs)), src.format().type());
+}
+
+// clang-format off
+#define FOREACH_IDENTITY_OP(cb) \
+    cb(Copy)                    \
+    cb(FastpathCopy)            \
+    cb(TypeCvt)                 \
+    cb(Pooling)                 \
+    cb(AdaptivePooling)         \
+    cb(Dropout)                 \
+    cb(Convolution)             \
+    cb(BatchNorm)               \
+    cb(Resize)                  \
+    cb(Identity)
+// clang-format on
+
+#define CREATE_IDENTITY_OP_RULE(op)                                            \
+    std::vector<ValueRef> op##_rule(                                           \
+            const op& _op, Span<ValueRef>& inputs, const bool& auto_convert) { \
+        return identity_rule_helper(_op, inputs);                              \
+    }
+FOREACH_IDENTITY_OP(CREATE_IDENTITY_OP_RULE)
+#undef CREATE_IDENTITY_OP_RULE
+
+#define REGISTER_IDENTITY_OP_RULE(op) register_format_rule(op##_rule);
+struct FormatRuleRegistry {
+    FormatRuleRegistry() {
+        register_format_rule(dimshuffle_rule);
+        register_format_rule(reshape_rule);
+        register_format_rule(broadcast_rule);
+        register_format_rule(subtensor_rule<Subtensor>);
+        register_format_rule(subtensor_rule<IndexingMultiAxisVec>);
+        register_format_rule(setsubtensor_rule<SetSubtensor>);
+        register_format_rule(setsubtensor_rule<IndexingSetMultiAxisVec>);
+        register_format_rule(concat_rule);
+        register_format_rule(elemwise_rule);
+        FOREACH_IDENTITY_OP(REGISTER_IDENTITY_OP_RULE)
+    }
+} _;
+#undef REGISTER_IDENTITY_OP_RULE
+}  // namespace
+
+std::vector<ValueRef> FormatTransformation::apply_transformation(
+        const Operator& op, Span<ValueRef> inputs) {
+    if (auto* apply_op = op.as<ApplyOp>()) {
+        // all inputs should be FormattedTensorValue
+        auto iter = format_rules.find(apply_op->op().dyn_typeinfo());
+        if (iter != format_rules.end()) {
+            return iter->second(apply_op->op(), inputs, m_auto_convert);
+        } else {
+            return wrap_outputs(imperative::apply(op, unwrap_inputs(inputs)));
+        }
+    } else if (auto* create_tensor = op.as<CreateTensor>()) {
+        auto format = create_tensor->format();
+        return {FormattedTensorValue::make(imperative::apply(op, inputs)[0], format)};
+    } else if (auto* get_attr = op.as<GetAttr>()) {
+        auto* src = inputs.as_array<1>()[0].as<FormattedTensorValue>();
+        if (!m_auto_convert || !src || src->format() != FT::NHWC) {
+            return imperative::apply(op, unwrap_inputs(inputs));
+        }
+        switch (get_attr->attr()) {
+            case GetAttr::Shape: {
+                auto output = imperative::apply(op, unwrap_inputs(inputs))[0];
+                auto shape = convert_nhwc2nchw_shape(output.cast<ShapeValue>());
+                return {ShapeValue::make(shape)};
+            }
+            case GetAttr::Value: {
+                auto nchw_src = unwrap_input(src->to(FT::NCHW, ""));
+                return imperative::apply(op, std::vector<ValueRef>{nchw_src});
+            }
+            default:
+                return imperative::apply(op, unwrap_inputs(inputs));
+        }
+    } else if (op.is<GetFormat>()) {
+        bool is_formatted_tensor = inputs.as_array<1>()[0].is<FormattedTensorValue>();
+        if (is_formatted_tensor) {
+            return {FormatValue::make(inputs[0].cast<FormattedTensorValue>().format())};
+        } else {
+            mgb_log_warn(
+                    "Not FormattedTensorValue input for GetFormat op: %s",
+                    inputs[0].to_string().c_str());
+            return {FormatValue::make(FT::DEFAULT)};
+        }
+    } else if (op.is<Operator::IdentityLike>()) {
+        bool is_formatted_tensor = inputs.as_array<1>()[0].is<FormattedTensorValue>();
+        if (is_formatted_tensor) {
+            auto& format = inputs[0].cast<FormattedTensorValue>().format();
+            return wrap_outputs(
+                    imperative::apply(op, unwrap_inputs(inputs)), format.type());
+        } else {
+            mgb_log_warn(
+                    "Not FormattedTensorValue input for IdentityLike op: %s",
+                    inputs[0].to_string().c_str());
+            return imperative::apply(op, inputs);
+        }
+    } else {
+        return imperative::apply(op, unwrap_inputs(inputs));
+    }
+};
+
+}  // namespace imperative
+}  // namespace mgb

imperative/src/impl/value.cpp

@@ -58,6 +58,10 @@ TypedValueRef<DTypeValue> ValueRef::dtype() const {
     return imperative::apply(GetAttr(GetAttr::DType), *this)[0].cast_ref<DTypeValue>();
 }
 
+TypedValueRef<FormatValue> ValueRef::format() const {
+    return imperative::apply(GetFormat(), *this)[0].as_ref<FormatValue>();
+}
+
 TypedValueRef<StringValue> ValueRef::name() const {
     return imperative::apply(GetName(), *this)[0].cast_ref<StringValue>();
 }

imperative/src/include/megbrain/imperative/basic_operators.h

@@ -5,6 +5,7 @@
 
 #include "megbrain/imperative/op_def.h"
 #include "megbrain/imperative/operator.h"
+#include "megbrain/imperative/utils/data_format.h"
 #include "megbrain/imperative/utils/helper.h"
 #include "megbrain/imperative/utils/value_shape.h"
 
@@ -82,9 +83,12 @@ private:
     CompNode m_device;
     DType m_dtype;
     ValueShape m_shape;
+    Format m_format;
 
 public:
-    CreateTensor(Kind kind, CompNode device, DType dtype, ValueShape shape);
+    CreateTensor(
+            Kind kind, CompNode device, DType dtype, ValueShape shape,
+            Format format = Format::Type::DEFAULT);
     CreateTensor(Kind kind, CompNode device, TensorLayout layout);
 
     /**
@@ -99,6 +103,7 @@ public:
     CompNode device() const { return m_device; }
     DType dtype() const { return m_dtype; }
     ValueShape shape() const { return m_shape; }
+    Format format() const { return m_format; }
 
     std::string to_string() const override;
 };
@@ -157,6 +162,11 @@ public:
     std::string to_string() const override;
 };
 
+class GetFormat final : public OperatorImpl<GetFormat, Operator::GetAttrLike> {
+public:
+    std::string to_string() const override { return "GetFormat{}"; }
+};
+
 class GetVarVal final : public OperatorImpl<GetVarVal, Operator::GetAttrLike> {
 public:
     std::string to_string() const override;

imperative/src/include/megbrain/imperative/basic_values.h

@@ -3,6 +3,7 @@
 #include <future>
 #include <iomanip>
 
+#include "megbrain/imperative/utils/data_format.h"
 #include "megbrain/imperative/utils/helper.h"
 #include "megbrain/imperative/utils/value_shape.h"
 #include "megbrain/imperative/value.h"
@@ -148,6 +149,13 @@ public:
     std::string to_string() const override;
 };
 
+class FormatValue final : public PrimitiveValue<FormatValue, Format> {
+public:
+    using PrimitiveValue::PrimitiveValue;
+
+    std::string to_string() const override { return Format::to_string(); }
+};
+
 class StringValue final : public PrimitiveValue<StringValue, std::string> {
 public:
     using PrimitiveValue::PrimitiveValue;

imperative/src/include/megbrain/imperative/transformations/format.h

@@ -0,0 +1,70 @@
+#pragma once
+
+#include "megbrain/imperative/basic_values.h"
+#include "megbrain/imperative/dispatch.h"
+#include "megbrain/imperative/ops/autogen.h"
+#include "megbrain/imperative/utils/data_format.h"
+
+namespace mgb::imperative {
+
+class FormattedTensorValue final : public ValueImpl<FormattedTensorValue> {
+private:
+    ValueRef m_value;
+    Format m_format;
+
+public:
+    FormattedTensorValue(ValueRef value, Format format)
+            : m_value(value), m_format(format) {}
+
+    std::string to_string() const override {
+        return ssprintf(
+                "FormattedTensorValue{value=%s, format=%s}",
+                m_value.to_string().c_str(), m_format.to_string().c_str());
+    }
+
+    ValueRef value() const { return m_value; }
+
+    const Format& format() const { return m_format; }
+
+    TypedValueRef<FormattedTensorValue> as(const Format::Type& target) const;
+    TypedValueRef<FormattedTensorValue> to(
+            const Format::Type& target, const std::string& scope = "") const;
+
+    void clear() override {
+        m_value = {};
+        m_format = {};
+    }
+
+    void on_watch() override { m_value.watch(); }
+
+    void on_unwatch() override { m_value.unwatch(); }
+};
+
+/**
+ * \brief simulates scalar because megbrain graph system don't support scalar
+ *
+ * Assume that we has 'a = ScalarValue(b)', thus 'a.shape == []', 'b.shape == [1]'.
+ * This transformation simulates scalars with a flag. If a value is ScalarValue, it is
+ * scalar, vice versa. So there is not scalar down this layer.
+ */
+class FormatTransformation final : public Transformation {
+private:
+    bool m_auto_convert = false;
+
+public:
+    std::vector<ValueRef> apply_transformation(
+            const Operator& op, Span<ValueRef> inputs) override;
+
+    ValueRef unwrap(ValueRef value) override {
+        mgb_assert(!value.is<FormattedTensorValue>());
+        return value;
+    }
+
+    std::string name() const override {
+        return ssprintf("FormatTransformation{auto_convert=%d}", m_auto_convert);
+    }
+    void set_auto_convert(bool enabled) { m_auto_convert = enabled; }
+    bool get_auto_convert() const { return m_auto_convert; }
+};
+
+}  // namespace mgb::imperative

imperative/src/include/megbrain/imperative/utils/data_format.h

@@ -0,0 +1,56 @@
+#pragma once
+
+#include "megbrain/tensor.h"
+
+namespace mgb::imperative {
+
+/**
+ * \brief like TensorFormats, but only including common formats and DEFAULT.
+ *
+ */
+class Format {
+public:
+    enum class Type {
+        DEFAULT = 0,
+        NCHW = 1,  ///< [N, C, H, W]
+        NHWC = 2,  ///< [N, H, W, C]
+    };
+    std::string to_string() const {
+        switch (m_type) {
+            case Type::DEFAULT:
+                return "default";
+            case Type::NCHW:
+                return "nchw";
+            case Type::NHWC:
+                return "nhwc";
+            default:
+                mgb_throw(MegBrainError, "bad format type");
+        }
+    }
+    Format() : m_type(Type::DEFAULT) {}
+    Format(std::string str) {
+        if (str == "default") {
+            m_type = Type::DEFAULT;
+        } else if (str == "nchw") {
+            m_type = Type::NCHW;
+        } else if (str == "nhwc") {
+            m_type = Type::NHWC;
+        } else {
+            mgb_throw(
+                    MegBrainError,
+                    "Invalid format type."
+                    " Only support \"default\", \"nchw\" and \"nhwc\"");
+        }
+    }
+    Format(Type type) : m_type(type) {}
+    Type type() const { return m_type; }
+    bool operator==(const Format& b) const { return m_type == b.type(); }
+    bool operator==(const Format::Type& b) const { return m_type == b; }
+    bool operator!=(const Format& b) const { return m_type != b.type(); }
+    bool operator!=(const Format::Type& b) const { return m_type != b; }
+
+private:
+    Type m_type = Type::DEFAULT;
+};
+
+}  // namespace mgb::imperative

imperative/src/include/megbrain/imperative/value.h

@@ -31,6 +31,7 @@ class HostValue;
 class DeviceValue;
 class ShapeValue;
 class DTypeValue;
+class FormatValue;
 class CompNodeValue;
 class StringValue;
 class NodeValue;
@@ -219,6 +220,7 @@ public:
     TypedValueRef<CompNodeValue> device() const;
     TypedValueRef<ShapeValue> shape() const;
     TypedValueRef<DTypeValue> dtype() const;
+    TypedValueRef<FormatValue> format() const;
     TypedValueRef<StringValue> name() const;
     bool is_scalar() const;
 
@@ -431,9 +433,11 @@ inline const TypedValueRef<TValue>& ValueRef::cast_ref(const Type<TValue>& type)
 inline void ValueRef::on_cast_failure(const IType& type) const {
     // if this is ErrorValue, rethrow directly
     storage()->try_rethrow();
-    mgb_assert(
-            storage()->type() != type, "expect type %s, got %s", type.name().c_str(),
-            to_string().c_str());
+    if (storage()->type() != type) {
+        mgb_throw(
+                MegBrainError, "Unable to cast ValueRef: expect type %s, got %s",
+                type.name().c_str(), to_string().c_str());
+    }
 }
 
 /**

src/opr/impl/dnn/batch_norm.cpp

@@ -200,7 +200,7 @@ void BatchNormForward::get_output_var_shape(
            bias_c = inp_shape[2][channel_idx];
     mgb_assert(
             inp_c == scale_c && inp_c == bias_c,
-            "inconsistent channel size, input chennel: %zu, scale channel: %zu, bias "
+            "inconsistent channel size, input channel: %zu, scale channel: %zu, bias "
             "channel: %zu",
             inp_c, scale_c, bias_c);