Fix model training state restoration in GRPO trainer

Store the model's training state before generation and restore inference
mode after completion if the model wasn't originally in training mode.
This ensures the model returns to the correct state after generate and
score operations.

unsloth/models/rl_replacements.py

@@ -259,6 +259,7 @@ def grpo_trainer__generate_and_score_completions(function_name, function):
     # The new multi-line string that will replace the line above
     replacement_lines = """
         batch_size = self.args.per_device_train_batch_size if mode == "train" else self.args.per_device_eval_batch_size
+        _was_training = self.model.training
         try:
             # TRL 0.23.1 and below path
             if not has_images:
@@ -387,6 +388,13 @@ def grpo_trainer__generate_and_score_completions(function_name, function):
             patched = patched[: match.start()] + wrapped + patched[match.end() :]
 
         function = patched
+    
+    function = function.replace(
+    "        return output",  # 8 spaces before 'return'
+    """        if not _was_training:
+            self.model.for_inference()
+        return output"""
+    )
 
     return function