Tensorflow 2.0 - Layer with fixed input
This feels like a bit of a hack, but I was able to infer something from an answer on another question: https://stackoverflow.com/a/46466275/6182971

It works if I change the first line above from:

ones = tf.ones(shape=(1,1))


to:

ones = layers.Lambda(lambda x: tf.ones(shape=(1,1)))(features_input)


Even though the Lambda layer is returning a constant, passing in features_input, which is the main training data connects the tf.ones constant to the network inputs, which seems to be sufficient.