[DDP Comm Hook] Use a single tensor instead of a tensor list as the comm hook result (#62074)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/62074
Since SPMD mode is retired, the comm hook result will always be a single tensor.
This can improve comm hook developer experience, as no need to add an extra `[0]` to the precursor future result.
#Closes: https://github.com/pytorch/pytorch/issues/61914
ghstack-source-id: 134164593
Test Plan:
buck test mode/dev-nosan caffe2/test/distributed:c10d
buck test mode/dev-nosan caffe2/test/distributed:distributed_nccl_fork
Reviewed By: rohan-varma
Differential Revision: D29864732
fbshipit-source-id: 59fe6dd78b66214b1788514ad4d236039d9bda31
