remove pytest.UsageError (#58916)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/58916
Using `pytest.UsageError` in case `pytest` is available adds almost
nothing as observed in
https://github.com/pytorch/pytorch/pull/53820#discussion_r593868752, but
makes it harder to maintain: due to the conditional import, `mypy` is
not able to handle `UsageError` in a type annotation.
Test Plan: Imported from OSS
Reviewed By: anjali411
Differential Revision: D29259409
Pulled By: mruberry
fbshipit-source-id: 82b00d13fa47db77383996d0caa69177804a48b6
