[optimizer] refactor RMSProp to use functional API (#50410)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/50410
Test Plan: Imported from OSS
Reviewed By: izdeby
Differential Revision: D25932779
Pulled By: wanchaol
fbshipit-source-id: b0d6007ea83d77e2d70d04681163ea7e4632c5cd
