使用pytorch grad cam时报错 ValueError: need at least one array to concatenate

使用pytorch grad cam时报错 ValueError: need at least one array to concatenate grad-cam计算时需要梯度信息，不能冻结模型权值…

如果冻结了权值，需要再设置回来

model.requires_grad_(True)

发生以下两种错误，应该都是这个原因

ValueError: need at least one array to concatenate
RuntimeError: element 0 of tensors does not require grad and does not have a grad_fn

官方代码&教程
pytorch-grad-cam