Config: warning when saving generation kwargs in the model config (#28514)

tests/test_modeling_utils.py

@@ -1230,6 +1230,15 @@ class ModelUtilsTest(TestCasePlus):
         for p1, p2 in zip(model.parameters(), new_model.parameters()):
             self.assertTrue(torch.equal(p1, p2))
 
+    def test_modifying_model_config_causes_warning_saving_generation_config(self):
+        model = AutoModelForCausalLM.from_pretrained("gpt2")
+        model.config.top_k = 1
+        with tempfile.TemporaryDirectory() as tmp_dir:
+            with self.assertLogs("transformers.modeling_utils", level="WARNING") as logs:
+                model.save_pretrained(tmp_dir)
+            self.assertEqual(len(logs.output), 1)
+            self.assertIn("Your generation config was originally created from the model config", logs.output[0])
+
 
 @slow
 @require_torch