*   (C++) `IteratorBase::Initialize()` method was added to support raising
        errors during iterator construction.
*   Eager Execution:
    *   Added the ability to pause recording operations for gradient computation
        via `tf.GradientTape.stop_recording`.
    *   Updated documentation, introductory notebooks.
*   `tf.keras`:
    *   Move Keras code out of _impl folder and remove API files.