Revert "Temporarily skip mypy-0.720 to unbreak master type checks"
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/23095
Test Plan: Imported from OSS
Differential Revision: D16383149
Pulled By: zou3519
fbshipit-source-id: ca6bdfe0f51f6bdbd4d95142a880f3902f60676d
