我试图训练一个代码，但我得到了这个错误。下面是我的代码：

for i in range(num_iter):
            # Forward pass to get logits
            logits = self(images)

            # Calculate loss
            loss = self.criterion(logits, masks)

            # Zero gradients
            self.optim.zero_grad()

            # Backward pass to compute gradients
            loss.backward()

            images = images.detach()

            images = torch.tensor(images, dtype=torch.float32, requires_grad=True)

            # Get the gradients of the loss w.r.t. the input image
            data_grad = images.grad.data

            # Generate the perturbed image using FGSM
            perturbed_images = fgsm(images, epsilon, data_grad)

            # Re-classify the perturbed image
            logits_perturbed = self(perturbed_images)

代码只是整个代码的一部分。
我看到的错误是：

File "/home/xx/xx/xx.py", line 73, in training_step
    data_grad = images.grad.data
AttributeError: 'NoneType' object has no attribute 'data'

尝试训练模型，期望它训练得很好，但它不起作用。