RuntimeError: element 0 of tensors does not require grad and does not have a grad_fn

[Rainfor1]

Hello, arnab39!
I have a problem for you.
When I run main.py, but I get this error:

Traceback (most recent call last):
File "main.py", line 88, in
main()
File "main.py", line 73, in main
model.train(args)
File "/home/lix/Documents/code/Semi-supervised-segmentation-cycleGAN/model.py", line 443, in train
gen_loss.backward()
File "/home/lix/miniconda3/envs/ptseg/lib/python3.6/site-packages/torch/tensor.py", line 107, in backward
torch.autograd.backward(self, gradient, retain_graph, create_graph)
File "/home/lix/miniconda3/envs/ptseg/lib/python3.6/site-packages/torch/autograd/init.py", line 93, in backward
allow_unreachable=True) # allow_unreachable flag
RuntimeError: element 0 of tensors does not require grad and does not have a grad_fn

How can I solve this problem?
I am looking forward for your reply!
Thanks!

[ReverseSystem001]

do you solve this problem now?