Set a rule on the config and split names
in this case we need to decide what to do with the existing datasets with white space characters (there shouldn't be a lot of them I think) I imagine that we should stop supporting them, and help the user fix them? See a report where the datasets server fails: https://huggingface.co/datasets/poloclub/diffusiondb/discussions/2#6374ff55b93cbdf65675f564

The config name is `random_10k [2m]`!
> should we actually allow characters like spaces? maybe it's better to add validation for whitespace symbols and directly in datasets and raise

https://github.com/huggingface/datasets-server/issues/853
