Add dataset hub.

[dirtycomputer]

Please add dataset hub, so that everyone can reuse the datasets someone created before.
And like hugging-face dataset, it can support everyone download dataset by just adding one-line code.
https://huggingface.co/datasets
https://huggingface.co/docs/datasets/index
And it can provide many dataset operations for medical 3D/2D images, medical texts and other modality data.

[wyli]

thanks, this PR (work in progress) might bring in the part of necessary capability for download/upload #6454

[dirtycomputer]

Maybe #6454 only add upload/download model parameters feature.
What I would like to say is that "please add dataset hub".