Build vocabulary.

def build(self):
        """
        Build vocabulary.
        """
        token_freq = self._token_count.most_common(self._max_size)
        idx = len(self.vocab)
        for token, _ in token_freq:
            self._token2id[token] = idx
            self._id2token.append(token)
            idx += 1
        if self._unk:
            unk = '<unk>'
            self._token2id[unk] = idx
            self._id2token.append(unk)