* [Improve] Speed up data preprocessor.
* Add ClsDataSample serialization override functions.
* Add unit tests
* Modify configs to fit new mixup args.
* Fix `num_classes` of the ImageNet-21k config.
* Update docs.
* add imagnet21k
* Update unit test
* update imaenet21k
* use slots
* use slots
* rename Data_item to ImageInfo
* add unit tests
* Update unit tests
* rm some print
* update unit tests
* fix lint
* remove default value of pipeline