singleRelu = (mat) => {
  // Only support square matrix
  console.assert(mat.length === mat[0].length, 'Activating non-square matrix!');

  let width = mat.length;
  let result = init2DArray(width, width, 0);

  for (let i = 0; i < width; i++) {
    for (let j = 0; j < width; j++) {
      result[i][j] = Math.max(0, mat[i][j]);
    }
  }
  return result;
}
Activate matrix mat using ReLU (max(0, x)).
@param {[[number]]} mat Matrix