Convert type annotations in nn/functional.py to py3 syntax (#53656)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/53656
Test Plan: Imported from OSS
Reviewed By: malfet
Differential Revision: D26926018
Pulled By: jamesr66a
fbshipit-source-id: 2381583cf93c9c9d0c9eeaa6e41eddce3729942d
