Urls for chunk: dict<str wiki_url, list<str> ref_urls>.

def _wiki_urls_for_shard(shard_id, urls_dir=None):
  """Urls for chunk: dict<str wiki_url, list<str> ref_urls>."""
  urls_dir = urls_dir or WIKI_URLS_DIR
  urls_filepath = os.path.join(urls_dir, WIKI_URLS_FILE % shard_id)
  with tf.gfile.GFile(urls_filepath) as f:
    return json.loads(f.read())