[BE] Introduce `set_cwd` context manager (#49657)
Summary:
Used to temporarily change working directory, but restore it even if exception is raised
Use it in test_type_hints and during code coverage collection
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49657
Reviewed By: walterddr
Differential Revision: D25660543
Pulled By: malfet
fbshipit-source-id: 77f08d57e4b60b95daa4068d0dacf7c25f978526
