Improved Documentation For Learning Rate Schedulers

[gabeorlanski]

Partially addresses #4835.

Changes proposed in this pull request:

• Improved documentation and a basic config snippet example for the learning rate schedulers
• Moved the Pytorch wrapped learning rates into their own file pytorch_lr_schedulers.py so that they have their own docs page
• Added links to the original PyTorch LRSchedulers so that people can read their docs as well.
• Added formula for Noam to its docs page.
• The only LR scheduler without an example is slanted_triangular.py as I have not used it before and do not know of a valid config for it.

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[AkshitaB]

This is excellent! Thank you for the contribution.