remove py2 flake8 lint (#29357)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29357
As title
Test Plan: Imported from OSS
Reviewed By: pritamdamania87
Differential Revision: D18920562
Pulled By: suo
fbshipit-source-id: b5dd559cfb0ba6c64b9ccf3655417afb56a7b472
