Fix None image handling in collate function

src/reasoning/step_level_cot.py

@@ -391,7 +391,11 @@ class StepLevelCoTTrainer:
                 text += f"Answer: {chain.final_answer}"
                 
                 texts.append(text)
-                images.append(chain.image)
+                # Handle missing images - append None which the processor should handle
+                if hasattr(chain, 'image'):
+                    images.append(chain.image if chain.image is not None else None)
+                else:
+                    images.append(None)
             
             # Tokenize with processor
             if hasattr(self.tokenizer, 'tokenizer'):
@@ -401,9 +405,13 @@ class StepLevelCoTTrainer:
                 
             # Use processor for vision-language models
             if hasattr(self.tokenizer, '__call__'):
+                # Filter out None images - if all are None, pass None instead of list
+                valid_images = [img for img in images if img is not None]
+                images_param = valid_images if valid_images else None
+                
                 inputs = self.tokenizer(
                     text=texts,
-                    images=images,
+                    images=images_param,
                     return_tensors="pt",
                     padding=True,
                     truncation=True,