First step of refactor lower passes (#74219)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/74219
X-link: https://github.com/pytorch/fx2trt/pull/18
This is beginning diff for refactor and clean up stuff in lowering process and pass management.
Test Plan: unit test
Reviewed By: yinghai
Differential Revision: D34764123
fbshipit-source-id: 57fa930abefae56654219225167b52d7dd79e03a
(cherry picked from commit 2ae868c83561c1a0dada4fab1b938f35d59ba3f8)
