I was working on a CNN model and I got this error without much information from the call stack. The only thing I know is It happened during backprop. Is there any common reason for this?
Call stack:
---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
<ipython-input-89> in <module>()
     29                 optimizer.zero_grad()
     30                 pred = model(data)
---> 31               criterion(pred, label).backward()
     32                 optimizer.step()
     33 

~/anaconda3/lib/python3.6/site-packages/torch/tensor.py in backward(self, gradient, retain_graph, create_graph)
     94                 products. Defaults to ``False``.
     95         """
---> 96         torch.autograd.backward(self, gradient, retain_graph, create_graph)
     97 
     98     def register_hook(self, hook):

~/anaconda3/lib/python3.6/site-packages/torch/autograd/__init__.py in backward(tensors, grad_tensors, retain_graph, create_graph, grad_variables)
     88     Variable._execution_engine.run_backward(
     89         tensors, grad_tensors, retain_graph, create_graph,
---> 90         allow_unreachable=True)  # allow_unreachable flag
     91 
     92