For the reference, the whole error message is as follows
Traceback (most recent call last):
  File "train_search.py", line 267, in <module>
    main()
  File "train_search.py", line 160, in main
    optimizer_weight, optimizer_arch, lr, train_arch=False, args=args)
  File "train_search.py", line 218, in train
    loss.backward()
  File "/home/ma-user/anaconda3/envs/Pytorch-1.0.0/lib/python3.6/site-packages/torch/tensor.py", line 102, in backward
    torch.autograd.backward(self, gradient, retain_graph, create_graph)
  File "/home/ma-user/anaconda3/envs/Pytorch-1.0.0/lib/python3.6/site-packages/torch/autograd/__init__.py", line 90, in backward
    allow_unreachable=True)  # allow_unreachable flag
  File "/home/ma-user/anaconda3/envs/Pytorch-1.0.0/lib/python3.6/site-packages/torch/autograd/function.py", line 76, in apply
    return self._forward_cls.backward(self, *args)
  File "/home/ma-user/work/ziqipang/darts-binary/models/mix_op.py", line 178, in backward
    detached_x, output = ctx.saved_tensors
RuntimeError: No grad accumulator for a saved leaf!