Add RMSprop optimizer to machine_learning - #14733

Describe your change:

Add RMSprop (Root Mean Square Propagation) optimizer implementation
to machine_learning. Complements the existing gradient_descent.py
with an adaptive learning rate optimizer.

Checklist:

• I have read CONTRIBUTING.md
• This is my own work, not plagiarized
• I know PRs won't merge if automated tests fail
• This PR only changes one algorithm file
• All new Python files are placed inside an existing directory