Enable torch.autograd typechecks (#44451)
Summary:
To help with further typing, move dynamically added native contributions from `torch.autograd` to `torch._C._autograd`
Fix invalid error handling pattern in
https://github.com/pytorch/pytorch/blob/89ac30afb88451efc66ed8077b7bbb222afd3428/torch/csrc/autograd/init.cpp#L13-L15
`PyImport_ImportModule` already raises Python exception and nullptr should be returned to properly propagate the to Python runtime.
And all native methods/types in `torch/autograd/__init.py` after `torch._C._init_autograd()` has been called
Use f-strings instead of `.format` in test_type_hints.py
Fixes https://github.com/pytorch/pytorch/issues/44450
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44451
Reviewed By: ezyang
Differential Revision: D23618261
Pulled By: malfet
fbshipit-source-id: fa5f739d7cff8410641128b55b810318c5f636ae
