Loss.backward() : RuntimeError: element 0 of tensors does not require grad and does not have a grad_fn

when I call loss.backward() I face the following problem:
RuntimeError: element 0 of tensors does not require grad and does not have a grad_fn
In my project (cloned from GitHub - geomagical/lama-with-refiner: 🦙 LaMa Image Inpainting, Resolution-robust Large Mask Inpainting with Fourier Convolutions, WACV 2022), I want to optimize an intermediate feature map while all network parameters remain unchanged (my network is pretrained).

This is my _infere function:

    masked_image = image * (1 - mask)
    masked_image = torch.cat([masked_image, mask], dim=1)
    mask = mask.repeat(1, 3, 1, 1)

    if ref_lower_res is not None:
        ref_lower_res = ref_lower_res.detach()
    with torch.no_grad():
        z1, z2 = forward_front(masked_image)
    # Inference
    mask = mask.to(device)
    ekernel = torch.from_numpy(cv2.getStructuringElement(cv2.MORPH_ELLIPSE, (15, 15)).astype(bool)).float()
    ekernel = ekernel.to(device)
    image = image.to(device)
    z1, z2 = z1.detach().to(device), z2.detach().to(device)
    z1.requires_grad, z2.requires_grad = True, True
    loss_fn = nn.L1Loss(size_average=None, reduce=None, reduction='mean')
    optimizer = Adam([z1, z2], lr=lr)
    pbar = tqdm(range(n_iters), leave=False)
    for idi in pbar:
        input_feat =(z1, z2)
        pred = forward_rear(input_feat)

        if ref_lower_res is None:
        # losses = {}
        # ------------------------- multi-scale -------------------------
        # scaled loss with downsampler
        pred_downscaled = _pyrdown(pred[:, :, :orig_shape[0], :orig_shape[1]])
        mask_downscaled = _pyrdown_mask(mask[:, :1, :orig_shape[0], :orig_shape[1]], blur_mask=False, round_up=False)
        mask_downscaled = _erode_mask(mask_downscaled, ekernel=ekernel)
        mask_downscaled = mask_downscaled.repeat(1, 3, 1, 1)

        loss = loss_fn(pred[mask < 1e-8], image[mask < 1e-8]) + \
               loss_fn(pred_downscaled[mask_downscaled >= 1e-8], ref_lower_res[mask_downscaled >= 1e-8])

            "Refining scale {} using scale {} ...current loss: {:.4f}".format(scale_ind + 1, scale_ind, loss.item()))
        if idi < n_iters - 1:
            optimizer.step()  # to optimize z
            del pred_downscaled
            del loss
            del pred

    # "pred" is the prediction after Plug-n-Play module
    inpainted = mask * pred + image * (1 - mask)
    inpainted = inpainted.detach().cpu()
    return inpainted

when I write loss.requires_grad = True before loss.backward, loss remains constant. I have checked other topics, but none of them were helpful.
What is the problem?
As I’ve noticed loss.backward calculates the gradient of all parameters for which we set required_grad= True. So is there any way to check which parameters have required_grad= True?

You are most likely detaching the loss from the computation graph e.g. by using a non-differentiable operation in the model or the loss calculation. Could you post a minimal, executable code snippet which would reproduce the issue, please?

Yes, this is expected as setting requires_grad=True on the loss would fix the runtime error, but will not fix the underlying error (detached computation graph).

Yes, you can iterate all parameters and check their attributes via:

for param in model.parameters():

Thank you so much for your quick response.
As the project is very complicated, I cannot reproduce the problem with a few lines. Therefore, I have debugged my code to show you the problem. This is the result. When I make prediction, the output is detached by default. Then when I use this output for calculating loss, the grad_fn of loss is None again.
Is there any way to attach a parameter manually?

Note: In this network , z is optimized.

As I feel the problem is caused by my model I send you the output of the debugging of my model. Here, we split our model into two parts, and use them form prediction.

No, that’s not possible. Once you’ve detached a tensor from the computation graph you could create a new graph but cannot reattach it.

In one of your screenshots you are explicitly executing the forward pass in a no_grad() context which would not create a computation graph at all, so you won’t be able to use backward() on this output or any resulting tensor created by this output.

You can post code snippets by wrapping them into three backticks ```, which would allow me to debug the issue as the screenshots are not really helpful in trying to isolate the error.

Thank you so much for your time and quick response.
I did not know with torch.no_grad the computation graph would not be created at all.
You helped me a lot. Thank you.