Add Python/C++ API parity tracker for torch.nn (#25289)
Summary:
This PR adds Python/C++ API parity tracker at `test/cpp_api_parity/parity-tracker.md`, which currently shows parity status for `torch.nn` modules.
A good amount of line changes here is moving `new_criterion_tests` from `test_nn.py` to `common_nn.py`, so that it can be used in `test_cpp_api_parity.py`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25289
Differential Revision: D17188085
Pulled By: yf225
fbshipit-source-id: 33d12fb1a4de2d9147ed09380973f361a3981fdf
