Make DatasetDict.column_names return a list instead of dict
Hi @albertvillanova, can I work on this issue?
Currently, `DatasetDict.column_names` returns a dict, with each split name as keys and the corresponding list of column names as values.

However, by construction, all splits have the same column names.

I think it makes more sense to return a single list with the column names, which is the same for all the split keys.