Download all FashionMNIST files to directory unless they are there.

def _get_fashion_mnist(directory):
  """Download all FashionMNIST files to directory unless they are there."""
  # Fashion mnist files have the same names as MNIST.
  # We must choose a separate name (by adding 'fashion-' prefix) in the tmp_dir.
  for filename in [
      _MNIST_TRAIN_DATA_FILENAME, _MNIST_TRAIN_LABELS_FILENAME,
      _MNIST_TEST_DATA_FILENAME, _MNIST_TEST_LABELS_FILENAME
  ]:
    generator_utils.maybe_download(directory,
                                   _FASHION_MNIST_LOCAL_FILE_PREFIX + filename,
                                   _FASHION_MNIST_URL + filename)