Allow a more backward compatible behavior of max_len_single_sentence and max_len_sentences_pair
Since #3706, `max_len_single_sentence` and `max_len_sentences_pair` are now automatically setup in the base class.

This PR allows the user to try to setup `max_len_single_sentence` and `max_len_sentences_pair`. If the value if the same as the pre-computed, we display a deprecation warning (avoid breaking old code in this case). If the value is different we raise an explicit error message.

cc @HendrikStrobelt