[DDP] add _distributed_rank helper function (#53795)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/53795
There are 4 calls in ddp implementation to dist.get_rank(), move these
to a helper property to ensure that users don't actually call `dist.get_rank()`
instead of `dist.get_rank(self.process_group)`.
Keeping API private for now because not sure if there is a user need to call `model.distributed_rank`, but can make it public if we think it's a useful api.
ghstack-source-id: 123640713
Test Plan: Ci
Reviewed By: mrshenli
Differential Revision: D26972368
fbshipit-source-id: a5f1cac243bca5c6f90a44f74d39cfffcc2b9a5a
