Fix ImageGPT doctest (#24353)

Fix doctest

src/transformers/models/imagegpt/modeling_imagegpt.py

@@ -980,7 +980,7 @@ class ImageGPTForCausalImageModeling(ImageGPTPreTrainedModel):
         >>> image_processor = AutoImageProcessor.from_pretrained("openai/imagegpt-small")
         >>> model = ImageGPTForCausalImageModeling.from_pretrained("openai/imagegpt-small")
         >>> device = torch.device("cuda" if torch.cuda.is_available() else "cpu")
-        >>> model.to(device)
+        >>> model.to(device)  # doctest: +IGNORE_RESULT
 
         >>> # unconditional generation of 8 images
         >>> batch_size = 4
@@ -1000,7 +1000,7 @@ class ImageGPTForCausalImageModeling(ImageGPTPreTrainedModel):
         ... ]  # convert color cluster tokens back to pixels
         >>> f, axes = plt.subplots(1, batch_size, dpi=300)
 
-        >>> for img, ax in zip(samples_img, axes):
+        >>> for img, ax in zip(samples_img, axes):  # doctest: +IGNORE_RESULT
         ...     ax.axis("off")
         ...     ax.imshow(img)
         ```"""