code: function addBias(arr) {
    if (!Array.isArray(arr) || !arr.length) {
      throw new Error(`Expected ${arr} to be an array with length`);
    } else if (!arr.every(Array.isArray)) {
      throw new Error(`Expected every item of ${arr} to be an array`);
    }
  
    return arr.map(row => [1].concat(row));
  }
docs: Add bias.
  
  @param {?} A
  @returns {Array}