make torch.testing asserts importable (#54769)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54769
Follow-up to #53820. This
- makes the `asserts.py` module private as per suggestion from rgommers in https://github.com/pytorch/pytorch/pull/53820#issuecomment-802661387. With this the functions should only be accessible through `torch.testing`, giving us the option the change the underlying structure later.
- moves the code from `torch/testing/__init__.py` to `torch/testing/_core.py` (happy to accept other name suggestions). Otherwise we can't import the new `_asserts.py` in `torch/testing/__init__.py` due to circular imports.
Test Plan: Imported from OSS
Reviewed By: mrshenli
Differential Revision: D27438451
Pulled By: mruberry
fbshipit-source-id: c7292b4d5709185b42b4aac8016648562688040e
