Returns SplitGenerators.

def _split_generators(self, dl_manager):
    """Returns SplitGenerators."""
    image_tar_file = os.path.join(dl_manager.manual_dir,
                                  self.builder_config.file_name)
    if not tf.io.gfile.exists(image_tar_file):
      # The current celebahq generation code depends on a concrete version of
      # pillow library and cannot be easily ported into tfds.
      msg = "You must download the dataset files manually and place them in: "
      msg += dl_manager.manual_dir
      msg += " as .tar files. See testing/test_data/fake_examples/celeb_a_hq "
      raise AssertionError(msg)
    return [
        tfds.core.SplitGenerator(
            name=tfds.Split.TRAIN,
            num_shards=50,
            gen_kwargs={"archive": dl_manager.iter_archive(image_tar_file)},
        )
    ]