[fx2trt]organize converter utils (#68015)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68015
Put all converter utils into a single file `converter_utils.py`.
Test Plan: CI
Reviewed By: wushirong
Differential Revision: D32250243
fbshipit-source-id: 93fb34bc9ca23f4c3cef3125e04871083dbd413d
