Revert D29701447: [DDP] Implement a hook which performs FunctionalSGD step.
Test Plan: revert-hammer
Differential Revision:
D29701447 (https://github.com/pytorch/pytorch/commit/bd95cf4473e4703c8b8a722f9c6e15ec9e7be8b4)
Original commit changeset: 183954593b82
fbshipit-source-id: 714e6a2b698147db9533a67783aed2a65d9d5bfe
