I’m getting the error *differentiated input is unreachable* on torch.autograd.grad in the following code. I’m using PyTorch 0.3.0, and everything runs fine up until the second to last line. Any help would be appreciated!

```
# forward
output = net(data)
# backward
loss = F.cross_entropy(output, target, reduce=False)
weights_normalized = weights / (weights.sum() + 1e-10)
loss = weights_normalized * loss
loss = loss.sum()
optimizer.zero_grad()
loss.backward(create_graph=True)
optimizer.step()
# forward val
output = net(data_val)
# backward val
loss = F.cross_entropy(output, target_val)
weights_grad = torch.autograd.grad(loss, weights, only_inputs=True)
weights = weights + 1e-2 * weights_grad
```