[Gradient Compression] Make GradBucket class public (#53099)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/53099
Publish GradBucket APIs for publishing DDP communication hooks.
s/_GradBucket/GradBucket
ghstack-source-id: 123030921
Test Plan: waitforbuildbot
Reviewed By: rohan-varma
Differential Revision: D26721121
fbshipit-source-id: ee5f68e33095b9965b51937b86cdeb331fd2419a
