[C++ API Parity] rmsprop optimizer update (#33450)
Summary:
**This PR is BC-breaking in the following way:**
In RMSpropOptions:
1. learning_rate is renamed to lr.
**Test plan before 1.5 release:**
Test that in 1.5 we can load a C++ RMSprop optimizer that was serialized in 1.4, and their states are the same.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33450
Differential Revision: D20366623
Pulled By: anjali411
fbshipit-source-id: 83250be9b583a766927e0e22a4de8b0765379451
