Load and normalize data.

def prepare_data():
  """Load and normalize data."""
  ((x_train, y_train), (x_test, y_test)) = DATASET.load_data()
  x_train = x_train.astype("float32")
  x_test = x_test.astype("float32")
  x_train /= 255.0
  x_test /= 255.0
  return ((x_train, y_train), (x_test, y_test))