Hi

Suppose we have a expression: z = x^2 *y + y, where x, y are both Tensors.

We want to calculate d(d z / d x) / dy. Here d is the partial derivative operator.

I do as follows:

```
import torch
x = torch.Tensor([3])
y = torch.Tensor([17])
x.requires_grad_()
y.requires_grad_()
loss = x**2 * y + y
loss.backward(create_graph=True, retain_graph=True)
xx = x.grad
xx.requires_grad_()
result = torch.autograd.grad(xx, y)
```

This piece of code can get correct result. However, I wander if this is the correct way to calculate, or I just get the correct result by coincidence.

Thanks!