[optim] make functional api be private (#51316)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51316
Make optim functional API be private until we release with beta
Test Plan: Imported from OSS
Reviewed By: albanD
Differential Revision: D26213469
fbshipit-source-id: b0fd001a8362ec1c152250bcd57c7205ed893107
