`.grad` instead of `.derivative` for tensors in `math_interface.py`

[jchwenger]

Hi there,

Spotted what looks like a small issue in math_interface.py:

                        oa.append((x, y, x1.derivative[0]))
                        ob.append((x, y, y1.derivative[0]))

I believe these ought to be:

                        oa.append((x, y, x1.grad[0]))
                        ob.append((x, y, y1.grad[0]))

(like here...)

Best,
Jeremie