Traceback (most recent call last):
File “train_search.py”, line 265, in 
main()
File “train_search.py”, line 179, in main
train_acc, train_obj = train(train_queue, valid_queue, model, architect, criterion, optimizer, lr)
File “train_search.py”, line 218, in train
architect.step(input, target, input_search, target_search, lr, optimizer, unrolled=args.unrolled)
File “/raid/shubhangi/Akshay/NAS/darts-depth/cnn/architect.py”, line 36, in step
self._backward_step(input_valid, target_valid)
File “/raid/shubhangi/Akshay/NAS/darts-depth/cnn/architect.py”, line 44, in _backward_step
loss.backward()
File “/home/cvprlab/.conda/envs/praful/lib/python3.5/site-packages/torch/tensor.py”, line 107, in backward
torch.autograd.backward(self, gradient, retain_graph, create_graph)
File “/home/cvprlab/.conda/envs/praful/lib/python3.5/site-packages/torch/autograd/init.py”, line 93, in backward
allow_unreachable=True)  # allow_unreachable flag
RuntimeError: element 0 of tensors does not require grad and does not have a grad_fn