[Docs] Add docs for BaseDataset (#7)

* add docs for BaseDataset

* update docs

* update docs

* update basedataset.md

* resolve all comments

* update

* update

* refine base dataset doc

* resolve all comments

* update

* update

* update

* update

* update

* update

* update

Co-authored-by: MeowZheng <meowzheng@outlook.com>