Add README to datapipes (#63982)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63982
Add a readme to `datapipes` for developer. This is can be a replacement of https://github.com/pytorch/pytorch/blob/master/torch/utils/data/datapipes_tutorial_dev_loaders.ipynb
After this PR is landed, the README.md will be added to PyTorch Wiki
Test Plan: Imported from OSS
Reviewed By: soulitzer
Differential Revision: D30554198
Pulled By: ejguan
fbshipit-source-id: 6091aae8ef915c7c1f00fbf45619c86c9558d308
