[DDP Comm Hook] Fix some comments (#61376)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61376
After SPMD is retired, the API of `get_tensors` becomes `get_tensor`. Fix some comments that refer to the obsolete API.
The `allreduce` hook example does not do division inside, which actually is incorrect.
ghstack-source-id: 133174272
Test Plan: N/A
Reviewed By: rohan-varma
Differential Revision: D29596857
fbshipit-source-id: 2046b185225cd6d1d104907b5f9b4009b6e87c99
