Fix DDP join() API in the case of model.no_sync() (#44427)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44427
Closes https://github.com/pytorch/pytorch/issues/44425
DDP join API currently does not work properly with `model.no_sync()`, see https://github.com/pytorch/pytorch/issues/44425 for details. This PR fixes the problem via the approach mentioned in the issue, namely scheduling an allreduce that tells joined ranks whether to sync in the backwards pass or not. Tests are added for skipping gradient synchronization for various `sync_interval`s.
ghstack-source-id: 111786479
Reviewed By: pritamdamania87
Differential Revision: D23609070
fbshipit-source-id: e8716b7881f8eee95e3e3499283e716bd3d7fe76
