control the inclusion of `Rescaling` layer in the model.
    *   Add optional argument (`force`) to `make_(train|test|predict)_funtion`
        methods to skip the cached function and generate a new one. This is
        useful to regenerate in a single call the compiled training function
        when any `.trainable` attribute of any model's layer has changed.
    *   Models now have a `save_spec` property which contains the `TensorSpec`