Turn `get_new_splits` into an abstract method
See https://github.com/huggingface/datasets-server/pull/839#issuecomment-1443264927. cc @AndreaFrancis 
Turn `get_new_splits` into an abstract method: See https://github.com/huggingface/datasets-server/pull/839#issuecomment-1443264927. cc @AndreaFrancis 