Add integration test for a generative RNN exported as SavedModel.

This tests the following:
 - @tf.function with and without input_signature,
 - several Keras layers including layers.LSTMCell, layers.RNN, layers.Dense,
 - capturing of a directly declared tf.Variable,
 - text preprocessing ops,
 - differentiation w.r.t. all of the above variables.

PiperOrigin-RevId: 231199593