Download and extract MSCOCO datasets to directory unless it is there.

def _get_mscoco(directory):
  """Download and extract MSCOCO datasets to directory unless it is there."""
  for url in _MSCOCO_URLS:
    filename = os.path.basename(url)
    download_url = os.path.join(_MSCOCO_ROOT_URL, url)
    path = generator_utils.maybe_download(directory, filename, download_url)
    unzip_dir = os.path.join(directory, filename.strip(".zip"))
    if not tf.gfile.Exists(unzip_dir):
      zipfile.ZipFile(path, "r").extractall(directory)