FBPFunctionGPU in leaptorch

How to solve this problem?The function is not correct.

class FBPFunctionGPU(torch.autograd.Function):
@staticmethod
def forward(ctx, input, proj, vol, param_id): # input: projection (sinogram), output: image
for batch in range(input.shape[0]):
f = vol[batch]
g = input[batch]
lct.fbp_gpu(g, f) # compute input (f) from proj (g)

    ctx.save_for_backward(input, proj, param_id)
    return vol
    
@staticmethod
def backward(ctx, grad_output): # grad_output: image, grad_input: projection (sinogram)
    input, proj, param_id = ctx.saved_tensors
    for batch in range(input.shape[0]):
        f = grad_output[batch]
        g = proj[batch]
        lct.fbp_adjoint_gpu(g, f) # compute proj (g) from input (f) -> needs to be replaced!!!
    return proj, None, None, None

What do I need to change to make this code correct?

Start with the FBPFunctionGPU forward and backward methods shown in the issue, then verify the expected autograd inputs and outputs for lct.fbp_gpu and lct.fbp_adjoint_gpu. Determine the correct adjoint operation and tensor handling before changing the implementation; done means the custom function computes the intended forward result and propagates correct gradients.