constructor(config) {
        super(config);
        // TODO use all of config: add_prefix_space, trim_offsets

        this.cls = config.cls[0];
        this.sep = config.sep[0];
    }
@param {Object} config The configuration for the post-processor.
@param {string[]} config.cls The special tokens to add to the beginning of the input.
@param {string[]} config.sep The special tokens to add to the end of the input.