Add huggingface dataset type

[martindurant]

No description provided.

[martindurant]

This isn't good enough, we need to be able to differentiate between a normal repo (for models) and dataset. Repo/model cards have a datasets, base_model, model_index keys; datasets may a key dataset_info: (containing features, download_size, dataset_size, splits).

That the models reference their datasets is most interesting.