Clean up simple type annotations in nn/functional.py (#50106)
Summary:
Also reformats code to pass linters.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50106
Test Plan: Sandcastle tests
Reviewed By: xush6528
Differential Revision: D25787566
fbshipit-source-id: 39c86b4021e279f92f8ccf30252a6cfae1063c3c
