For serving/predict, assume that only video frames are provided.

def serving_input_fn(self, hparams):
    """For serving/predict, assume that only video frames are provided."""
    video_input_frames = tf.placeholder(
        dtype=tf.float32,
        shape=[
            None, hparams.video_num_input_frames, self.frame_width,
            self.frame_height, self.num_channels
        ])

    # TODO(michalski): add support for passing input_action and input_reward.
    return tf.estimator.export.ServingInputReceiver(
        features={"inputs": video_input_frames},
        receiver_tensors=video_input_frames)