Create subgraph rewriter (#49540)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/49540
Test Plan: Imported from OSS
Reviewed By: pbelevich
Differential Revision: D25869707
Pulled By: ansley
fbshipit-source-id: 93d3889f7ae2ecc5e8cdd7f4fb6b0446dbb3cb31
