[optim] refactor rprop to use functional API (#55832)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55832
ghstack-source-id: 126325541
Reviewed By: driazati
Differential Revision: D27703877
fbshipit-source-id: 34d4ce7b7d124c0cd75e2f6d0bc8f836713b7301
