Add a section to introduce GradBucket class in ddp_comm_hooks.rst (#53253)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/53253
Since GradBucket class becomes public, mention this class in ddp_comm_hooks.rst.
Screenshot:
{F478201008}
ghstack-source-id: 123596842
Test Plan: viewed generated html file
Reviewed By: rohan-varma
Differential Revision: D26812210
fbshipit-source-id: 65b70a45096b39f7d41a195e65b365b722645000
