[torch.fx] Add support for custom op (#43248)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43248
We add the support of __torch_function__ override for C++ custom op. The logic is the same as the other components, like torch.nn.Module.
Refactored some code a little bit to make it reusable.
Test Plan: buck test //caffe2/test:fx -- test_torch_custom_ops
Reviewed By: bradleyhd
Differential Revision: D23203204
fbshipit-source-id: c462a86e407e46c777171da32d7a40860acf061e
