[ao][sparsity] README for base data scheduler class (#82131)
The readme file contains an overview of the base data scheduler.
Consists of code snippets and instructions on how to create your own custom
data scheduler and how to use during training a model.
Test Plan: None
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82131
Approved by: https://github.com/z-a-f
