Make testing._internal.common_utils safe to import (#99659)
In edge cases in CI, SLOW_TESTS_FILE is defined but does not point to an existing file.
Guessing this is due to a test case that opens a subprocses and cwd's but doesn't clean its env.
We shouldn't make importing common_utils fail, so issue a warning and proceed.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/99659
Approved by: https://github.com/ezyang, https://github.com/malfet
