*   `tf.data`
    *   Makes the following (breaking) changes to the `tf.data`.
    *   C++ API: - `IteratorBase::RestoreInternal`,
        `IteratorBase::SaveInternal`, and `DatasetBase::CheckExternalState`
        become pure-virtual and subclasses are now expected to provide an
        implementation.
    *   The deprecated `DatasetBase::IsStateful` method is removed in favor of
        `DatasetBase::CheckExternalState`.