[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
