fix flake8 (#19243)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19243
ghimport-source-id: ae80aed3a5742df21afb6e55979686220a27cce7
Differential Revision: D14928670
Pulled By: zdevito
fbshipit-source-id: 20ec0d5c8d6f1c515beb55e2e63eddf3b2fc12dd
