Hello, I am trying to train a CycleGAN model in PyTorch. Following is the code snippet for training the two generators alternately:

```
# Training G_ST and G_TS
if epoch % 5 == 0:
optimizer_G_ST.zero_grad()
L_GAN_ST_forGST = - torch.mean(torch.log(D_T_G_ST_E_x_s_forGST))
L_cyc_forGST = cycleLoss(G_ST_G_TS_E_x_t, E_x_t)
L_GAN_ST = L_GAN_ST_forGST + L_cyc_forGST
L_GAN_ST.backward(retain_graph=True)
optimizer_G_ST.step()
if epoch % 5 == 1:
optimizer_G_TS.zero_grad()
L_GAN_TS_forGTS = - torch.mean(torch.log(D_S_G_TS_E_x_t_forGTS))
L_cyc_forGTS = cycleLoss(G_TS_G_ST_E_x_s, E_x_s)
L_GAN_TS = L_GAN_TS_forGTS + L_cyc_forGTS
L_GAN_TS.backward()
optimizer_G_TS.step()
```

For the line of code `L_GAN_ST.backward(retain_graph=True)`

, I am getting the following error:

`RuntimeError: Trying to backward through the graph a second time (or directly access saved tensors after they have already been freed). Saved intermediate values of the graph are freed when you call .backward() or autograd.grad(). Specify retain_graph=True if you need to backward through the graph a second time or if you need to access saved tensors after calling backward.`

What is the reason for this error and how do I resolve this? A few answers have suggested using `retain_graph=True`

which I tried but it is not working. Please help.