Not the class itself (which is already a callable), but an instance of that class.

To do that, we declare a method `__call__`:

{* ../../docs_src/dependencies/tutorial011_an_py39.py hl[12] *}

    Note that this feature is only available with Python 3.7 or higher.

    *   Improve the informativeness of error messages raised by Keras
        `Layer.__call__()`, by adding the full list of argument values passed to
        the layer in every exception.

*   Introduce the `tf.compat.v1.keras.utils.track_tf1_style_variables`