[Model Averaging] Code simplification for _find_process_group function (#75007)
Summary:
Previously the highest-level process group in `period_process_group_dict` could be `None`, indicating the global group. Now `period_process_group_dict` cannot contain `None` as a process group, so the function `_find_process_group` can just return a process group instead of a tuple -- when not found, just return `None`, because now the returned process group cannot be `None`.
Proposal: https://github.com/pytorch/pytorch/issues/71325
Pull Request resolved: https://github.com/pytorch/pytorch/pull/75007
Reviewed By: awgu
Differential Revision: D35357816
Pulled By: rohan-varma
fbshipit-source-id: 4522dba49797df7140227bfd822d668b7e118a66
(cherry picked from commit 77ca01b555d52685283c969176b08de4ff46c32d)
