Add Python/C++ torch.nn API parity test harness (#23852)
Summary:
This PR adds test harness for checking Python / C++ API parity for `torch.nn.Module` subclasses. Under the hood, we use JIT tracing to transfer `nn.Module` state from Python to C++, so that we can test initialization / forward / backward on Python / C++ modules with the same parameters and buffers.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/23852
Differential Revision: D16830204
Pulled By: yf225
fbshipit-source-id: 9b5298c0e8cd30e341a9f026e6f05604a82d6002
