Read pre-trained embedding file for extending vocabulary

        Parameters
        ----------
        pret_embeddings : tuple
            (embedding_name, source), used for gluonnlp.embedding.create(embedding_name, source)

def _add_pret_words(self, pret_embeddings):
        """Read pre-trained embedding file for extending vocabulary

        Parameters
        ----------
        pret_embeddings : tuple
            (embedding_name, source), used for gluonnlp.embedding.create(embedding_name, source)
        """
        words_in_train_data = set(self._id2word)
        pret_embeddings = gluonnlp.embedding.create(pret_embeddings[0], source=pret_embeddings[1])

        for idx, token in enumerate(pret_embeddings.idx_to_token):
            if token not in words_in_train_data:
                self._id2word.append(token)