RuntimeError: Assertion `cur_target >= 0 && cur_target < n_classes' failed
Okay after 4 days I got my solution eventually. I figured it out that my classes are started from 1 to 12, so, in total I had 12 classes. 

The problem was appearing when I was about to calculate the predicted class vs actual class loss with the loss function. Instead of having start from 1 that function always from class 0 instead of 1 and continuously look for 0 to 12 which indicated it do have 13 classes in total which generates an exception. 

So how do I solved that? I just docked 1 form each labels and shifted it from 0 to 11 and it went fine. That's how I solved the issue. Mention, I got couple of solution at PyTorch forum regarding the same issue and those wasn't helpful at all, literally!