Solved by ptrblck in post #8 
              
              
                Oh, it seems I’ve missed an important line in the stack trace. 
File "train.py", line 40, in <module>
    model = ColorizationCycleGAN(args)

Could you check your definition of ColorizationCycleGAN? 
I would always recommend to update to the latest release, but this error seems unrelated to your PyT…