[ao][sparsity] README for BaseDataSparsifier (#82130)
The readme file contains an overview of the base data sparsifier, it's implementation details.
Also, consists of code snippets and instructions on how to create your own custom
data sparsifier.
Test Plan: None
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82130
Approved by: https://github.com/z-a-f
