Pytorch训练时报错Target 255 is out of bounds.

问题遇到的现象和发生背景

UNet网络使用Pytorch训练时报错Target 255 is out of bounds.

问题相关代码，请勿粘贴截图

:\Anaconda\Anaconda\envs\PyTorch\lib\site-packages\torch\autocast_mode.py:162: UserWarning: User provided device_type of 'cuda', but CUDA is not available. Disabling
  warnings.warn('User provided device_type of \'cuda\', but CUDA is not available. Disabling')
Epoch 1/5:   0%|                                                                                                             | 0/5 [00:04<?, ?img/s]
Traceback (most recent call last):
  File "c:\Users\18497\Downloads\Pytorch-UNet-master\Pytorch-UNet-master\train.py", line 185, in <module>
    train_net(net=net,
  File "c:\Users\18497\Downloads\Pytorch-UNet-master\Pytorch-UNet-master\train.py", line 93, in train_net
    loss = criterion(masks_pred, true_masks) \
  File "D:\Anaconda\Anaconda\envs\PyTorch\lib\site-packages\torch\nn\modules\module.py", line 1110, in _call_impl
    return forward_call(*input, **kwargs)
  File "D:\Anaconda\Anaconda\envs\PyTorch\lib\site-packages\torch\nn\modules\loss.py", line 1163, in forward
    return F.cross_entropy(input, target, weight=self.weight,
  File "D:\Anaconda\Anaconda\envs\PyTorch\lib\site-packages\torch\nn\functional.py", line 2996, in cross_entropy
    return torch._C._nn.cross_entropy_loss(input, target, weight, _Reduction.get_enum(reduction), ignore_index, label_smoothing)
IndexError: Target 255 is out of bounds.

我的解答思路和尝试过的方法

我想要达到的结果

我想请问一下，这个Target 255是代表我设定的类别有255类吗，有关类别的代码模块一般在什么位置呀？我使用自己的数据集但是不知道有哪些地方需要修改代码里的参数ww，dl帮帮我看看，答疑解惑,谢谢！！