code: function tril(A, k) {
      if (k === void 0) { k = 0; }
      if (A.shape.length !== 2) {
          throw new Error('Input is not matrix');
      }
      var copyA = A.clone();
      for (var i = 0; i < copyA.shape[0]; i++) {
          for (var j = 0; j < copyA.shape[1]; j++) {
              if (i < j - k) {
                  copyA.set(i, j, 0);
              }
          }
      }
      return copyA;
  }
docs: Create Lower triangular matrix from given matrix