[DDP] Refactor uneven inputs to take GradBucket (#61019)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61019
Changes uneven input logic of running allreduce to using `GradBucket` structure. This is to enable support for comm. hook with join in the next diff.
ghstack-source-id: 132950107
Test Plan: ci
Reviewed By: SciPioneer
Differential Revision: D29480027
fbshipit-source-id: 7c42c53653052f71b86a75e14a5fc7ae656433f7
