Can't pass `sep=None` to load_dataset("csv", ...) to infer the separator via pandas.read_csv
When doing `load_dataset("csv", sep=None)`, the `sep` passed to `pd.read_csv` is still the default `sep=","` instead, which makes it impossible to make the csv loader infer the separator.

Related to https://github.com/huggingface/datasets/pull/2656

cc @SBrandeis 