Remove Inspector API usage from lib.py (#327)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/327

Revert what I did to lib.py in D48456830. Reasoning:

1. We redesigned the Inspector interface in D49099840, so the Inspector won't work with the lib.py on master anymore;
2. We initially wanted to invoke Inspector from lib.py, but then later decided to have the user interact with Inspector directly.

Users will be able to enter a CLI debugging flow from the Inspector directly. Once that's implemented and landed, we can then work on removing duplicated CLI features from lib.py.

Reviewed By: Jack-Khuu

Differential Revision: D49249836

fbshipit-source-id: 7d5a8a59764e639db699c2f3a52723d112698841