UserWaring:The .grad attribute of a Tensor thatis not a leaf Tensor is being accessed.Its .grad attribute won’t be populated during autograd.backward().If you Indeed want the .grad field to be populated for a non-leaf Tensor,use .retain_grad() on the non-leaf Tensor.If you access the non-leaf Tensor by mistake ,make sure you access the leaf Tensor instead.See github.com/pytorch/pytorch/pull/30531 for more information
在print(y.grad)这一句报错，注释了就没错了
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