refactor fx2trt example script so it can be imported as a library (#63262)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63262
Just create a `__main__` guard.
Test Plan: run linter, sandcastle tests
Reviewed By: 842974287
Differential Revision: D30263617
fbshipit-source-id: 8044ce5d815b043c3778591384cb13d9a89d0048
