Returns dict<str ref_url, str ref_content>.

def _references_content(ref_files):
  """Returns dict<str ref_url, str ref_content>."""
  example_spec = {
      "url": tf.FixedLenFeature([], tf.string),
      "content": tf.FixedLenFeature([], tf.string),
  }
  data = {}
  for ex in generator_utils.tfrecord_iterator(
      ref_files, gzipped=True, example_spec=example_spec):
    data[ex["url"]] = text_encoder.to_unicode(ex["content"])
  return data