`tf.keras.optimizers.Optimizer`/`Adam`/etc will point to the new
        optimizers, and the previous generation of optimizers will be moved to
        `tf.keras.optimizers.legacy.Optimizer`/`Adam`/etc.
    *   Added L2 unit normalization layer `tf.keras.layers.UnitNormalization`.
    *   Added `tf.keras.regularizers.OrthogonalRegularizer`, a new regularizer
        that encourages orthogonality between the rows (or columns) or a weight