Tensorflow version needs to be pinned to <2.16 or model needs to updated to Keras 3

[Wiebke]

The model makes use of Keras 2, but TensorFlow installs Keras 3 from version 2.16 onwards.

An updated version of the referenced example can be found under https://github.com/keras-team/keras-io/blob/master/examples/generative/ddim.py, and considerations regarding Keras 3 are listed under https://keras.io/keras_3/