*   Added `tf.keras.optimizers.experimental.Optimizer`. The reworked
        optimizer gives more control over different phases of optimizer calls,
        and is easier to customize. We provide Adam, SGD, Adadelta, AdaGrad and
        RMSprop optimizers based on
        `tf.keras.optimizers.experimental.Optimizer`. Generally the new
        optimizers work in the same way as the old ones, but support new