implementation.
    *   The deprecated `DatasetBase::IsStateful` method is removed in favor of
        `DatasetBase::CheckExternalState`.
    *   Deprecated overrides of `DatasetBase::MakeIterator` and
        `MakeIteratorFromInputElement` are removed.
    *   The signature of `tensorflow::data::IteratorBase::SaveInternal` and
        `tensorflow::data::IteratorBase::SaveInput` has been extended with