[Optimizer Overlap] Move hooks to own file (#71601)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/71601
Moves current prototype optimizer overlap to its own file for a better
namespace. No code changes besides a few comment fixes. Note that this code is
still prototype and not expected to be used by an end user.
ghstack-source-id: 147458826
Test Plan: CI
Reviewed By: cbalioglu
Differential Revision: D33662678
fbshipit-source-id: 3cc931323230a4b66c02b9e6f744aaf5c48d4d34
(cherry picked from commit 5070595c7f6de85f75249eb22cbd561f9450fcc2)
