RuntimeError: One of the differentiated Tensors appears to not have been used in the graph. Set allow_unused=True if this is the desired behavior. srun: error: cn31: task 0: Exited with exit code 1

How can i solve this error?
RuntimeError: One of the differentiated Tensors appears to not have been used in the graph. Set allow_unused=True if this is the desired behavior.
srun: error: cn31: task 0: Exited with exit code 1

for batch_idx, (x_in,y_in) in enumerate(dataloader):
                    net2_u.zero_grad()
                    net2_v.zero_grad()
                    net2_p.zero_grad()
                    loss_eqn = criterion(x_in,y_in)
                    loss_bc = Loss_BC(xb,yb,ub,vb,x,y)
                    loss = loss_eqn + Lambda_BC* loss_bc
                    # loss.backward(create_graph=True)
                    loss.backward()
                    optimizer_u.step() 
                    optimizer_v.step() 
                    optimizer_p.step()
                    loss_eqn_a =loss_eqn.detach().cpu().numpy()
                    loss_eqn_n += loss_eqn_a
                    loss_bc_a= loss_bc.detach().cpu().numpy()
                    loss_bc_n += loss_bc_a  
                    n += 1         
                    
                    
                    
                    params, gradsH = get_params_grad(net2_u) #get model parameters and corresponding gradients
                
                    params[2].requires_grad = True
                    gradsH[2].requires_grad = True
                    tmp_num_data = x_in.size(0)
                    
                    v = [torch.randn(p.size()).to(device) for p in params]  # generate random vector
                    v = normalization(v)  # normalize the vector
                    THv = [torch.zeros(p.size()) for p in params[2]]
                    Hv = torch.autograd.grad(gradsH[2],params[2],grad_outputs=torch.ones_like(params[2]), retain_graph = True ,only_inputs=True)[0]
                  

If i set allow_unused=True it prints none.