* [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.
* [Refactor] Refactor the rest data transforms and fix some docstring.
* Use `mmengine` instead of `mmcv`
* Refactor KFold dataset tools
* Fix docstring according to comments