Example:
  >>> M = torch.randn(2, 3)
  >>> mat1 = torch.randn(2, 3)
  >>> mat2 = torch.randn(3, 3)
  >>> torch.addmm(M, mat1, mat2)
  tensor([[-4.8716,  1.4671, -1.3746],
          [ 0.7573, -3.9555, -2.8681]])
