Add flake8 via lintrunner (#241)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/241

Made sure it is consistent with Meta internal FLAKE8 rules (https://fb.workplace.com/groups/pythonlinters/posts/684192797083133)

Will gradually add more and more rules and linters

Reviewed By: digantdesai, dbort

Differential Revision: D49069290

fbshipit-source-id: 3990d3af80fe67bc6e5dfa2ae0221bacd6d2f5b7