fix(mge): fix layer norm amp bug

GitOrigin-RevId: dba691fcbf

imperative/python/megengine/functional/nn.py

@@ -1090,6 +1090,9 @@ def layer_norm(
         eps_mode
     )
 
+    if amp._enabled:
+        inp, weight, bias = cast_tensors(inp, weight, bias, promote=True)
+
     _device = inp.device
     _dtype = inp.dtype
     _dim = len(inp.shape) - len(normalized_shape)