Introduce API `tff.learning.models.functional_model_from_keras`.
This is a method to create a `tff.learning.models.FunctionalModel` from a `tf.keras.Model`. PiperOrigin-RevId: 411169398
This is a method to create a `tff.learning.models.FunctionalModel` from a `tf.keras.Model`. PiperOrigin-RevId: 411169398