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

[LLL-YUE]

While running one_hot.backward(retain_graph=True) , an error appears.
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.
But retain_graph=True is already in backward()
How can I deal with this situation?