I have a puzzle about the torch autograd. When I build a model, and write code:

```
x.requires_grad = True
pred = model(x)
pred.sum().backward()
print(x.grad)
```

I can get the grad of x. I want to go forward from here. Is there a method that I can get d(x.sum())/d(pred) using the following code?

```
x.requires_grad = True
pred = model(x)
pred.requires_grad = True
pred.sum().backward(retain_graph=True)
x.sum().backward()
print(pred.grad)
```

I have tried this, but the pred.grad is None. and x.grad.grad_fn is None. Thanks for your help