Pythorh：Trying to backward through the graph a second time, but the buffers have already been freed

错误：

Traceback (most recent call last):
  File "train.py", line 419, in 
    main()
  File "train.py", line 409, in main
    segm_crit1, segm_crit2)
  File "train.py", line 275, in train_segmenter
    loss2.backward()
  File "/home/wit/anaconda2/envs/pytorch_27/lib/python2.7/site-packages/torch/tensor.py", line 102, in backward
    torch.autograd.backward(self, gradient, retain_graph, create_graph)
  File "/home/wit/anaconda2/envs/pytorch_27/lib/python2.7/site-packages/torch/autograd/__init__.py", line 90, in backward
    allow_unreachable=True)  # allow_unreachable flag
RuntimeError: Trying to backward through the graph a second time, but the buffers have already been freed. Specify retain_graph=True when calling backward the first time.

解决：[参考1]