I am a beginner and I have been following tutorials and tying them out in my own jupyter notebook. I make a lot of mistakes with syntax so I change my code a lot and rerun them a lot in my notebook. Because of this I have been constantly running into Trying to backward through the graph a second time" Error because I edited my code so much. I have no idea how to fix this, the first good result calls the use of retain_graph=True within .backward() but I have tried to add that parameter and my notebook still shows the same error. How do I clear my graph so I can rerun my code block without encountering this issue?

My simple code block

```
x1 = torch.Tensor( [ 1, 2, 3, 4 ])
x1_var = Variable( x1, requires_grad= True )
target_y = Variable(torch.Tensor([0]), requires_grad=False)
linear_layer1 = nn.Linear(4, 1)
loss_function = nn.MSELoss()
TOTAL_EPOCHS = 5
LEARNING_RATE = 0.001
optimizer = optim.SGD(linear_layer1.parameters(), lr=LEARNING_RATE)
# Note we need to update linear_layer1 's weights
for epochs in range(TOTAL_EPOCHS):
linear_layer1.zero_grad()
predicted_y = linear_layer1(x1_var)
loss = loss_function(predict_y, target_y)
loss.backward()
optimizer.step()
print("------------------------------------------")
print("x1_var")
```

Setting .backward(retain_graph=True) does not work, I still get the same error. How do you fix this?