To add single and chained learning schedulers to docs (#56705)
Summary:
In the optimizer documentation, many of the learning rate schedulers [examples](https://pytorch.org/docs/stable/optim.html#how-to-adjust-learning-rate) are provided according to a generic template. In this PR we provide a precise simple use case example to show how to use learning rate schedulers. Moreover, in a followup example we show an example how to chain two schedulers next to each other.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56705
Reviewed By: ezyang
Differential Revision: D27966704
Pulled By: iramazanli
fbshipit-source-id: f32b2d70d5cad7132335a9b13a2afa3ac3315a13
