Revert D26070147: [Gradient Compression] Refactor default_hooks.py and powerSGD_hook.py by creating a util function that make a vanilla allreduce future
Test Plan: revert-hammer
Differential Revision:
D26070147 (https://github.com/pytorch/pytorch/commit/e7b34962326713aa65383c509c7939f2a007211b)
Original commit changeset: 8c9339f1511e
fbshipit-source-id: fa1e9582baec9759a73b3004be9bb19bdeb6cd34
