[DDP] Remove python GradBucket construction (#60301)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/60301
`GradBucket` is not meant to be constructed by Python user, only
consumed as part of comm. hook
ghstack-source-id: 131860243
Test Plan: CI
Reviewed By: SciPioneer
Differential Revision: D29239320
fbshipit-source-id: f1631a16e7d66b7e4a9b4a44698e2319005d10b2
