[fx2trt] fix import in oss tests (#68016)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68016
We would want to use oss test utils.
Also refactor both test utils so that the internal one is an enhancement over the oss test utils.
Test Plan: CI
Reviewed By: wushirong
Differential Revision: D32250266
fbshipit-source-id: 968b8f215ca2d294f7d0bd13cf9563be567954dd
