def tf_reference(self, states, internals, reward, update):
return None<EOL>
Creates the TensorFlow operations for obtaining the reference tensor(s), in case of a
comparative loss.

Args:
    states: Dict of state tensors.
    internals: List of prior internal state tensors.
    reward: Reward tensor.
    update: Boolean tensor indicating whether this call happens during an update.

Returns:
    Reference tensor(s).