[Gradient Compression] Add is_the_last_bucket_to_allreduce method to GradBucket class (#53010)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/53010
To determine the boundary between different iterations in a DDP communication hook, currently the user code needs `bucket.get_index() == 0`, which involves internal bucketization implementation details and undermines the usability of DDP communication hook.
Create an API to hide the details and improve the usability before publishing GradBucket APIs.
ghstack-source-id: 122723081
Test Plan: buck test mode/dev-nosan caffe2/test/distributed:c10d -- test_powerSGD_ddp_comm_hook_nccl
Reviewed By: rohan-varma
Differential Revision: D26720813
fbshipit-source-id: f4a3147382c1f970534d7f0dee0cd599156c8b8c
