Get filenames for a particular split.

def _subset_filenames(dl_paths, split):
  """Get filenames for a particular split."""
  assert isinstance(dl_paths, dict), dl_paths
  # Get filenames for a split.
  if split == tfds.Split.TRAIN:
    urls = _get_url_hashes(dl_paths['train_urls'])
  elif split == tfds.Split.VALIDATION:
    urls = _get_url_hashes(dl_paths['val_urls'])
  elif split == tfds.Split.TEST:
    urls = _get_url_hashes(dl_paths['test_urls'])
  else:
    logging.fatal('Unsupported split: %s', split)
  cnn = _find_files(dl_paths, 'cnn', urls)
  dm = _find_files(dl_paths, 'dm', urls)
  return cnn + dm