pytorch

5a962369 - [Gradient Compression] Check if the backend is NCCL when a DDP communication hook is registered (#51759)

[Gradient Compression] Check if the backend is NCCL when a DDP communication hook is registered (#51759) Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/51759 Some unit tests actually register a comm hook on other backends like GLOO. Example: `test_ddp_comm_hook_future_passing_cpu` Therefore, only do the check on `register_builtin_comm_hook`. Currently DDP communication hook can only be supported on NCCL. Add a check in the registration methods. ghstack-source-id: 121115814 Test Plan: unit tests. Reviewed By: pritamdamania87 Differential Revision: D26268581 fbshipit-source-id: c739fa4dca6d320202dc6689d790c2761c834c30