[Gradient checkpoining] Correct disabling find_unused_parameters in Trainer when gradient checkpointing is enabled (#13961)

* up

* correct test

tests/test_modeling_common.py

@@ -197,6 +197,25 @@ class ModelTesterMixin:
                 )
                 self.assertTrue(len(load_result.unexpected_keys) == 0)
 
+    def test_gradient_checkpointing_enable_disable(self):
+        config, inputs_dict = self.model_tester.prepare_config_and_inputs_for_common()
+
+        for model_class in self.all_model_classes:
+            if not model_class.supports_gradient_checkpointing:
+                continue
+
+            # at init model should have gradient checkpointing disabled
+            model = model_class(config)
+            self.assertFalse(model.is_gradient_checkpointing)
+
+            # check enable works
+            model.gradient_checkpointing_enable()
+            self.assertTrue(model.is_gradient_checkpointing)
+
+            # check disable works
+            model.gradient_checkpointing_disable()
+            self.assertFalse(model.is_gradient_checkpointing)
+
     def _mock_init_weights(self, module):
         if hasattr(module, "weight") and module.weight is not None:
             module.weight.data.fill_(3)