MegEngine/python_module/megengine/quantization/fake_quant.py

# MegEngine is Licensed under the Apache License, Version 2.0 (the "License")
#
# Copyright (c) 2014-2020 Megvii Inc. All rights reserved.
#
# Unless required by applicable law or agreed to in writing,
# software distributed under the License is distributed on an
# "AS IS" BASIS, WITHOUT ARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
import copy
import math

import numpy as np

from .. import functional as F
from .._internal.dtype import _metadata_dict, get_quantized_dtype
from ..core import Buffer, Function, Parameter
from ..jit import sideeffect
from ..module import Module
from .observer import ObserverMode, Round


class _FakeQuantize(Module):
    def __init__(self, dtype: str, enable: bool = True):
        super().__init__()
        if not dtype in _metadata_dict.keys():
            raise ValueError(
                "unknown dtype: {}, only support {}".format(
                    dtype, _metadata_dict.keys()
                )
            )
        self.dtype = dtype
        self.qmin = _metadata_dict[dtype].qmin
        self.qmax = _metadata_dict[dtype].qmax
        self.enabled = enable

    def enable(self):
        self.enabled = True

    def disable(self):
        self.enabled = False

    def fake_quant_forward(self, inp, q_dict):
        return inp

    def normal_foward(self, inp, q_dict):
        return inp

    def forward(self, inp, q_dict):
        if self.enabled:
            return self.fake_quant_forward(inp, q_dict)
        else:
            return self.normal_foward(inp, q_dict)


class TQT_Function(Function):
    def __init__(self, lowerbound, upperbound):
        super().__init__()
        self.lowerbound = lowerbound
        self.upperbound = upperbound

    def forward(self, inp, scale):
        t = 2 ** scale
        # t = F.maximum(t, 1e-4)
        inp_scaled = inp / t
        inp_clipped = F.maximum(F.minimum(inp_scaled, self.upperbound), self.lowerbound)
        inp_rounded = F.round(inp_clipped)
        inp_flq = inp_rounded * t
        self.save_for_backward(inp_scaled, inp_rounded, t)
        return inp_flq

    def backward(self, grad_inp_flq):
        (inp_scaled, inp_rounded, t) = self.saved_tensors
        mask_clip = (inp_scaled < -0.5 + self.lowerbound) + (
            inp_scaled > self.upperbound + 0.5
        )  # mask for accumulating the gradients of |data_scaled|>L
        mask_quant = F.abs(
            mask_clip - 1
        )  # mask for accumulating the gradients with |data_scaled|<=L
        grad_quant = (
            grad_inp_flq * mask_quant * (inp_rounded - inp_scaled)
        )  # gradient within |data_scaled|<=L
        grad_clip = (
            grad_inp_flq * mask_clip * inp_rounded
        )  # gradient with   | data_scaled|>L
        grad_s = grad_clip.sum() + grad_quant.sum()
        # dL/ds = dL/dt * t * ln(2)
        grad_s = grad_s * t * math.log(2)
        grad_inp = grad_inp_flq * mask_quant
        return grad_inp, grad_s


class TQT(_FakeQuantize):
    """
    TQT: https://arxiv.org/abs/1903.08066 Trained Quantization Thresholds 
    for Accurate and Efficient Fixed-Point Inference of Deep Neural Networks
    """

    def __init__(self, dtype: str, enable: bool = True):
        super().__init__(dtype, enable)
        self.scale = Parameter(0.0, dtype=np.float32)

    def fake_quant_forward(self, inp, q_dict):
        # when enable, TQT will do fakequant forward, finetune the scale
        return TQT_Function(self.qmin, self.qmax)(inp, self.scale)

    def normal_foward(self, inp, q_dict):
        # when disable, TQT will do normal forward, initialize scale weight
        tmp_scale = F.maximum(F.abs(q_dict["min_val"]), F.abs(q_dict["max_val"]))
        tmp_scale = F.log(tmp_scale / 127) / F.log(2)
        F.add_update(self.scale, tmp_scale, alpha=0.0, beta=1.0, bias=0.0)
        return inp

    def get_dtype(self):
        return get_quantized_dtype(self.dtype, 2 ** self.scale.numpy()[0], None)


class FakeQuantize(_FakeQuantize):
    r"""
    A module to do quant and dequant according to observer's scale and zero_point.
    """

    def fake_quant_forward(self, inp, q_dict):
        if q_dict["mode"] == ObserverMode.SYMMERTIC:
            scale = q_dict["scale"]
            # Quant
            oup = Round()(inp / scale)
            # clip
            oup = F.minimum(F.maximum(oup, self.qmin), self.qmax)
            # DeQuant
            oup = (oup) * scale
            return oup
        else:
            scale = q_dict["scale"]
            zero_point = q_dict["zero_point"]
            # Quant
            oup = Round()(inp / scale) + zero_point
            # clip
            oup = F.minimum(F.maximum(oup, self.qmin), self.qmax)
            # DeQuant
            oup = (oup - zero_point) * scale
            return oup