[DDP] Remove SPMD from get_bucket_tensors (#61017)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61017
Removes SPMD nested vector logic from this codepath. This is mostly in preparation for the next diffs in this stack which enable support for join with comm. hook.
ghstack-source-id: 132924223
Test Plan: CI
Reviewed By: SciPioneer
Differential Revision: D29477360
fbshipit-source-id: f8132a94b1abfe28586aa78ac47e13a7ce6bb137
