def vectorize_target(self, size):
        """
        Make targets be one-hot vectors.
        """
        if self._train_set:
            self._train_set = self._vectorize_set(self._train_set, size)
        if self._valid_set:
            self._valid_set = self._vectorize_set(self._valid_set, size)
        if self._test_set:
            self._test_set = self._vectorize_set(self._test_set, size)
Make targets be one-hot vectors.