Solved by ptrblck in post #3 
              
              
                You are detaching the loss tensors by wrapping them into the deprecated Variable in: 
D_loss = Variable((D_fake_loss + D_real_loss)/2, requires_grad=True)

so you should remove the Variable usage. 
(Note that the same issue would be caused, if you rewrap the loss in a new tensor.) 
Also, if you are …