code: def _wiki_urls_for_shard(shard_id, urls_dir=None):
    
    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())
docs: Urls for chunk: dict<str wiki_url, list<str> ref_urls>.