Use github actions for flake8 (#25824)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25824
Use github actions for flake8. This is nice because it makes it easier
to create inline annotations for lint violations.
It ends up looking like this:
https://github.com/suo/pytorch/pull/21/files
Test Plan: Imported from OSS
Differential Revision: D17487007
Pulled By: suo
fbshipit-source-id: 663094ea2bbbdb1da5b7e5d294c70735a319d5e5
