import tensorflow as tf


  def example_reading_spec(self):
    data_fields = {"targets": tf.VarLenFeature(tf.int64)}
    if self.has_inputs:
      data_fields["inputs"] = tf.VarLenFeature(tf.int64)

    if self.packed_length:
      if self.has_inputs:
        data_fields["inputs_segmentation"] = tf.VarLenFeature(tf.int64)
        data_fields["inputs_position"] = tf.VarLenFeature(tf.int64)
      data_fields["targets_segmentation"] = tf.VarLenFeature(tf.int64)
      data_fields["targets_position"] = tf.VarLenFeature(tf.int64)

    data_items_to_decoders = None
    return (data_fields, data_items_to_decoders)

  def eval_metrics(self):
    return [
