Extract TIMIT datasets to directory unless directory/timit exists.

def _get_timit(directory):
  """Extract TIMIT datasets to directory unless directory/timit exists."""
  if os.path.exists(os.path.join(directory, "timit")):
    return

  assert FLAGS.timit_paths
  for path in FLAGS.timit_paths.split(","):
    with tf.gfile.GFile(path) as f:
      with tarfile.open(fileobj=f, mode="r:gz") as timit_compressed:
        timit_compressed.extractall(directory)