[DDP] Make uneven inputs work with comm. hook (#61020)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61020
Makes uneven input support with `join` context manager work with
custom communication hooks. This will ensure that the two features can work
well together. Added relevant unittests to test allreduce and powerSGD hooks.
Instead of calling `allreduce`, the join manager now calls into `_run_reduction_hook` which will automatically run whatever hook is installed.
ghstack-source-id: 132950108
Test Plan: CI
Reviewed By: SciPioneer
Differential Revision: D29480028
fbshipit-source-id: c91dc467a62c5f1e0ec702a2944ae3deb10f93f4
