[DataPipe] Fixes various warnings, exceptions, and clean up testing (#81833)
I went through most of the warnings and exceptions raised in our tests to find these issues.
Changes:
1. In testing, `self.assertEquals` is deprecated, converting to `self.assertEqual` to get rid of the warning
2. Small changes for cleanliness and get rid of warnings (no actual change to result)
3. Correct `is_every_instance_exhausted` logic for `_Forker`
4. Catch `RunTimeError` raised by invalidated iterator during clean up
5. Check if attribute `parent_stream` exists before trying to access it
Differential Revision: [D38020122](https://our.internmc.facebook.com/intern/diff/D38020122)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81833
Approved by: https://github.com/ejguan
