`get_dataset` fails with some

Example: the loading of `GEM/wiki_lingua` subset `id` in promptsource fails, even though calling `datasets.load_dataset("GEM/wiki_lingua", "id")` works perfectly.
I suspect we can remove the logic with `builder_instance` and just use `datasets.load_dataset`

https://github.com/bigscience-workshop/promptsource/blob/d1f16cfa786ee496588de7117abe5ab6c5326f67/promptsource/utils.py#L36