To add Rprop documentation (#63866)

To add Rprop documentation (#63866)

Summary:
It has been discussed before that adding description of Optimization algorithms to PyTorch Core documentation may result in a nice Optimization research tutorial. In the following tracking issue we mentioned about all the necessary algorithms and links to the originally published paper  https://github.com/pytorch/pytorch/issues/63236.

In this PR we are adding description of Rprop to the documentation.  For more details, we refer to the paper  http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.21.1417

<img width="657" alt="Rpropalg" src="https://user-images.githubusercontent.com/73658284/132750009-a5ec059e-6d53-4c67-917b-57174c8ca27b.png">

Pull Request resolved: https://github.com/pytorch/pytorch/pull/63866

Reviewed By: ngimel

Differential Revision: D30867590

Pulled By: iramazanli

fbshipit-source-id: 0d2d4ffc6c4d939290bbbaa84d2c6e901ed8b54a