[DDP Communication Hook] Update the typing info of comm hook output as well as some docstring (#62457)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/62457
Specify `Future[torch.Tensor]` as DDP communication hook return type, which should be explicitly a single tensor. The previous API takes a list that has a single tensor.
Note that now the typing info no longer accepts the internal type of `torch._C.Future`, which does not support torchscript and hence cannot support `Future[torch.Tensor]`.
ghstack-source-id: 134771419
Test Plan:
buck test mode/dev-nosan caffe2/test/distributed:c10d -- test_default_ddp_comm_hooks_nccl
buck test mode/dev-nosan caffe2/test/distributed:c10d -- test_ddp_invalid_comm_hook_return_type
Reviewed By: rohan-varma
Differential Revision: D30007390
fbshipit-source-id: 246667c9b575b4c6e617b0a5b373151f1bd81e7f
