[PEFT] Set eval mode when loading PEFT adapter (#34509)

* [PEFT] Set eval mode when loading PEFT adapter

Resolves #34469

When calling model.load_adapter to load a PEFT adapter, by default the
adapter should be set to eval mode. This is now correctly done. Users
can still pass is_trainable=True to load the adapter in training mode.

* Linter

tests/peft_integration/test_peft_integration.py

@@ -622,3 +622,46 @@ class PeftIntegrationTester(unittest.TestCase, PeftTesterMixin):
 
                 msg = f"Loading adapter weights from state_dict led to missing keys in the model: {key}"
                 self.assertIn(msg, cl.out)
+
+    def test_peft_load_adapter_training_inference_mode_true(self):
+        """
+        By default, when loading an adapter, the whole model should be in eval mode and no parameter should have
+        requires_grad=False.
+        """
+        for model_id in self.peft_test_model_ids:
+            for transformers_class in self.transformers_test_model_classes:
+                peft_model = transformers_class.from_pretrained(model_id).to(torch_device)
+
+                with tempfile.TemporaryDirectory() as tmpdirname:
+                    peft_model.save_pretrained(tmpdirname)
+                    model = transformers_class.from_pretrained(peft_model.config._name_or_path)
+                    model.load_adapter(tmpdirname)
+                    assert not any(p.requires_grad for p in model.parameters())
+                    assert not any(m.training for m in model.modules())
+                    del model
+
+    def test_peft_load_adapter_training_inference_mode_false(self):
+        """
+        When passing is_trainable=True, the LoRA modules should be in training mode and their parameters should have
+        requires_grad=True.
+        """
+        for model_id in self.peft_test_model_ids:
+            for transformers_class in self.transformers_test_model_classes:
+                peft_model = transformers_class.from_pretrained(model_id).to(torch_device)
+
+                with tempfile.TemporaryDirectory() as tmpdirname:
+                    peft_model.save_pretrained(tmpdirname)
+                    model = transformers_class.from_pretrained(peft_model.config._name_or_path)
+                    model.load_adapter(tmpdirname, is_trainable=True)
+
+                    for name, module in model.named_modules():
+                        if len(list(module.children())):
+                            # only check leaf modules
+                            continue
+
+                        if "lora_" in name:
+                            assert module.training
+                            assert all(p.requires_grad for p in module.parameters())
+                        else:
+                            assert not module.training
+                            assert all(not p.requires_grad for p in module.parameters())