Collator fix

Author: jakep-allenai

olmocr/train/train.py

@@ -68,7 +68,7 @@ def __call__(self, examples):
             'input_ids': torch.stack(batch['input_ids']),
             'attention_mask': torch.stack(batch['attention_mask']),
             'labels': torch.stack(batch['labels']),
-            'pixel_values': batch['pixel_values'],  # Keep as list for now
+            'pixel_values': torch.stack(batch['pixel_values']),  # Stack into tensor
             'image_grid_thw': torch.stack(batch['image_grid_thw'])
         }