Download (and unzip) a file from the MNIST dataset if not already done.

def download(directory, filename):
  """Download (and unzip) a file from the MNIST dataset if not already done."""
  filepath = os.path.join(directory, filename)
  if tf.gfile.Exists(filepath):
    return filepath
  if not tf.gfile.Exists(directory):
    tf.gfile.MakeDirs(directory)
  url = 'http://yann.lecun.com/exdb/mnist/' + filename + '.gz'
  _, zipped_filepath = tempfile.mkstemp(suffix='.gz')
  print('Downloading %s to %s' % (url, zipped_filepath))
  urllib.request.urlretrieve(url, zipped_filepath)
  with gzip.open(zipped_filepath, 'rb') as f_in, \
      tf.gfile.Open(filepath, 'wb') as f_out:
    shutil.copyfileobj(f_in, f_out)
  os.remove(zipped_filepath)
  return filepath