* PPIDataset
* Revert "PPIDataset"
This reverts commit 264bd0c960cfa698a7bb946dad132bf52c2d0c8a.
* Hide implementations that may cause confusions to the user
* fix number of classes in Amazon Cobuy datasets
* add four papers
* Fix doc string
* fix readme
* fix readme
Co-authored-by: Quan (Andy) Gan <coin2028@hotmail.com>