[SPMD] Restrict DDP communication hooks to SPSD mode (#55253)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55253
Previously DDP communication hooks takes a tensor list as the input. Now only takes a single tensor, as the preparation of retiring SPMD and only providing a single model replica for DDP communication hooks.
The next step is limiting only 1 model replica in Reducer.
ghstack-source-id: 125677637
Test Plan: waitforbuildbot
Reviewed By: zhaojuanmao
Differential Revision: D27533898
fbshipit-source-id: 5db92549c440f33662cf4edf8e0a0fd024101eae
