[reland] move rebuild buckets from end of first iteration to beginning of second iteration (#44798)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44798
[test all]
Update for relanding: in ddp.join(), moved _rebuild_buckets from end of backward to beginning of forward as well.
Part of relanding PR #41954, this refactoring is to move rebuild_buckets call from end of first iteration to beginning of second iteration
ghstack-source-id: 112279261
ghstack-source-id: 112279261
Test Plan: unit tests
Reviewed By: rohan-varma
Differential Revision: D23735185
fbshipit-source-id: c26e0efeecb3511640120faa1122a2c856cd694e
