Clean up onnxifi transformation code (#15453)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15453
Just move things around to facilitate further development. No logic change.
Reviewed By: rdzhabarov
Differential Revision: D13533959
fbshipit-source-id: eebab1306939e802aacffb24a711d372fd67916c
