Keras 3.3.3 upgrade

[Chase-Grajeda]

• Included Keras 3.3.3 upgrades for all network classes and instances of tf.model, tf.layer, and tf.optimizer
• Added @tf.function decorator to coupling_networks._calculate_spline() to allow execution in non-eager environments
• Changed instances of tf.Tensor in docstring to Tensor w.r.t. the experimental Tensor type

The included changes have worked successfully in all of the example / tutorial notebooks, however I'm sure there are a few introduced bugs laying around when TensorFlow is not the chosen backend.

Current environment dependencies:

• keras 3.3.3
• tensorflow 2.16.1
• tensorflow-intel 2.16.1
• tensorflow-probability 0.24.0

[LarsKue]

We cannot merge this anymore due to the many changes on the branch, but we'll use this as a reference nonetheless.