def extra_reading_spec(self):
    """Additional data fields to store on disk and their decoders."""
    data_fields = {
        "frame_number": tf.FixedLenFeature([1], tf.int64),
    }
    decoders = {
        "frame_number":
            contrib.slim().tfexample_decoder.Tensor(tensor_key="frame_number"),
    }
    return data_fields, decoders
Additional data fields to store on disk and their decoders.
tensor2tensor/data_generators/moving_mnist.py