code: function stringifySlotSizes(sizes) {
    let result = '';
    if (utils.isArray(sizes)) {
      result = sizes.reduce((acc, size) => {
        acc.push(size.join('.'));
        return acc;
      }, []);
      result = '[' + result.join(',') + ']';
    }
    return result;
  }
docs: Converts GPT-style size array into a string
  @param  {Array} sizes:  list of GPT-style sizes, e.g. [[300, 250], [300, 300]]
  @return {String} a string containing sizes, e.g. '[300.250,300.300]'