Setting a 1-D feature with shape=(0,) builds datasets.Sequence(length=0, ...),
which pyarrow rejects with ArrowInvalid: list_size needs to be a strict
positive integer when datasets.Dataset.from_dict(...) is called inside
save_episode. Use length=-1 (variable-length) for zero-width 1-D shapes.
Fixes the second half of #3654 (the first half is #3664, in compute_episode_stats).
Mahbod