e.g.`tf.image.ssim_multiscale`
    *   Error messages when Functional API construction goes wrong (and when ops
        cannot be converted to Keras layers automatically) should be clearer and
        easier to understand.
*   `Optimizer.minimize` can now accept a loss `Tensor` and a `GradientTape` as
    an alternative to accepting a `callable` loss.
*   Adds `beta` hyperparameter to