Compute stats for Sequence feature only if it's underlying schema is List
Should fix https://huggingface.co/datasets/OpenAssistant/oasst1 (see "emojis" and "label" columns).
When sequence feature is dict of lists (not list of dicts), compute lengths over first subfield values.  
Compute stats for Sequence feature only if it's underlying schema is List: Should fix https://huggingface.co/datasets/OpenAssistant/oasst1 (see "emojis" and "label" columns).
When sequence feature is dict of lists (not list of dicts), compute lengths over first subfield values.  