post_process(tokens, tokens_pair = null) {
        if (tokens_pair) {
            tokens = mergeArrays(tokens, tokens_pair);
        }
        return { tokens };
    }
Post process the given tokens.
@param {string[]} tokens The list of tokens for the first sequence.
@param {string[]} [tokens_pair=null] The list of tokens for the second sequence (optional).
@returns {PostProcessedOutput} An object containing the post-processed tokens.