I wish to update my PolicyNet network based on a loss from two variables not on the graph. So, any idea how I can only update PolicyNet based on a loss that does not require grad? Thanks.

```
actions = PolicyNet.forward(state)
loss = criterion(a,b) # a and b have requires_grad=False
loss = Variable(loss, requires_grad=True)
loss.backward()
print(list(PolicyNet.parameters())[0].grad)
optimizer.step()
```