Add a description of GradBucket Python class (#53596)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/53596
This description will be used in ddp_comm_hook docstrings.
ghstack-source-id: 123590360
Test Plan: waitforbuildbot
Reviewed By: rohan-varma
Differential Revision: D26908160
fbshipit-source-id: 824dea9203ca583676bddf0161c9edca52c9d20e
