compare_results() implementation (including matplotlib ploting) (#1347)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1347

As titled, also amended the existing CLI to include a compare_results flag. When the flag is true, we call compare_results() on the reference output and run output saved in the event block named "Execute".

Reviewed By: tarun292, Jack-Khuu

Differential Revision: D51823130

fbshipit-source-id: 648edc14d61b9455b33876e0f1d0c74909eaa1ec