Additional data fields to store on disk and their decoders.

def extra_reading_spec(self):
    """Additional data fields to store on disk and their decoders."""
    field_names = ("frame_number", "action", "reward", "done")
    data_fields = {
        name: tf.FixedLenFeature([1], tf.int64) for name in field_names
    }
    decoders = {
        name: tf.contrib.slim.tfexample_decoder.Tensor(tensor_key=name)
        for name in field_names
    }
    return (data_fields, decoders)