Traceback (most recent call last):                                                
  File "tmp.py", line 63, in <module>                                             
    loss.backward()                                                               
  File "/scratch/apaszke/pytorch/torch/tensor.py", line 96, in backward           
    torch.autograd.backward(self, gradient, retain_graph, create_graph)           
  File "/scratch/apaszke/pytorch/torch/autograd/__init__.py", line 90, in backward
    allow_unreachable=True)  # allow_unreachable flag                             
RuntimeError: undefined Tensor