add assert for fb_overwritable decorator.

Summary:
X-link: https://github.com/facebookresearch/mobile-vision/pull/108

Pull Request resolved: https://github.com/facebookresearch/d2go/pull/373

Asserting that methods replaced by fb_overwritable are also annotated with corresponding decorator.

Reviewed By: wat3rBro

Differential Revision: D39674347

fbshipit-source-id: a4cf007419760aa07d929ab1cf819ba54b11b9da

d2go/utils/helper.py

@@ -136,7 +136,6 @@ def get_dir_path(relative_path):
 
 
 # copy util function for oss
-@fb_overwritable()
 def alias(x, name, is_backward=False):
     if not torch.onnx.is_in_onnx_export():
         return x