Remove _compatible_subtest (#35620)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35620
Python 2 has reached end-of-life and is no longer supported by PyTorch.
`self.subTest` can be used directly in Python 3.
Test Plan: CI
Differential Revision: D20842872
Pulled By: dreiss
fbshipit-source-id: 6ad42550c01e6959821ff07df767fc14b58c5a9e
