Downloads all lsun files to directory unless they are there.

def _get_lsun(directory, category, split_name):
  """Downloads all lsun files to directory unless they are there."""
  generator_utils.maybe_download(directory,
                                 _LSUN_DATA_FILENAME % (category, split_name),
                                 _LSUN_URL % (category, split_name))