tox.ini -> .flake8 (#15065)
Summary:
We were only using this file to configure flake8, and fbcode linters do not recognize tox.ini which causes spurious linter warnings.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15065
Differential Revision: D13420774
Pulled By: suo
fbshipit-source-id: e43a46befa36862c8b3c0a90074aec6a66531492
