[PEFT] Fix PEFT + gradient checkpointing (#25846)

* fix PEFT + gradient checkpointing

* add disable RG

* polish tests

* fix comment

* Revert "fix comment"

This reverts commit b85386f50d.

* final explanations and tests

src/transformers/modeling_utils.py

@@ -1750,6 +1750,13 @@ class PreTrainedModel(nn.Module, ModuleUtilsMixin, GenerationMixin, PushToHubMix
             raise ValueError(f"{self.__class__.__name__} does not support gradient checkpointing.")
         self.apply(partial(self._set_gradient_checkpointing, value=True))
 
+        if getattr(self, "_hf_peft_config_loaded", False):
+            # When using PEFT + gradient checkpointing + Trainer we need to make sure the input has requires_grad=True
+            # we do it also on PEFT: https://github.com/huggingface/peft/blob/85013987aa82aa1af3da1236b6902556ce3e483e/src/peft/peft_model.py#L334
+            # When training with PEFT, only LoRA layers will have requires grad set to True, but the output of frozen layers need to propagate
+            # the gradients to make sure the gradient flows.
+            self.enable_input_require_grads()
+
     def gradient_checkpointing_disable(self):
         """
         Deactivates gradient checkpointing for the current model.
@@ -1760,6 +1767,9 @@ class PreTrainedModel(nn.Module, ModuleUtilsMixin, GenerationMixin, PushToHubMix
         if self.supports_gradient_checkpointing:
             self.apply(partial(self._set_gradient_checkpointing, value=False))
 
+        if getattr(self, "_hf_peft_config_loaded", False):
+            self.disable_input_require_grads()
+
     @property
     def is_gradient_checkpointing(self) -> bool:
         """

tests/peft_integration/test_peft_integration.py

@@ -179,6 +179,52 @@ class PeftIntegrationTester(unittest.TestCase, PeftTesterMixin):
                     model_from_pretrained = transformers_class.from_pretrained(tmpdirname).to(torch_device)
                     self.assertTrue(self._check_lora_correctly_converted(model_from_pretrained))
 
+    def test_peft_add_adapter_training_gradient_checkpointing(self):
+        """
+        Simple test that tests if `add_adapter` works as expected when training with
+        gradient checkpointing.
+        """
+        from peft import LoraConfig
+
+        for model_id in self.transformers_test_model_ids:
+            for transformers_class in self.transformers_test_model_classes:
+                model = transformers_class.from_pretrained(model_id).to(torch_device)
+
+                peft_config = LoraConfig(init_lora_weights=False)
+
+                model.add_adapter(peft_config)
+
+                self.assertTrue(self._check_lora_correctly_converted(model))
+
+                # When attaching adapters the input embeddings will stay frozen, this will
+                # lead to the output embedding having requires_grad=False.
+                dummy_input = torch.LongTensor([[0, 1, 2, 3, 4, 5, 6, 7]]).to(torch_device)
+                frozen_output = model.get_input_embeddings()(dummy_input)
+                self.assertTrue(frozen_output.requires_grad is False)
+
+                model.gradient_checkpointing_enable()
+
+                # Since here we attached the hook, the input should have requires_grad to set
+                # properly
+                non_frozen_output = model.get_input_embeddings()(dummy_input)
+                self.assertTrue(non_frozen_output.requires_grad is True)
+
+                # To repro the Trainer issue
+                dummy_input.requires_grad = False
+
+                for name, param in model.named_parameters():
+                    if "lora" in name.lower():
+                        self.assertTrue(param.requires_grad)
+
+                logits = model(dummy_input).logits
+                loss = logits.mean()
+                loss.backward()
+
+                for name, param in model.named_parameters():
+                    if param.requires_grad:
+                        self.assertTrue("lora" in name.lower())
+                        self.assertTrue(param.grad is not None)
+
     def test_peft_add_multi_adapter(self):
         """
         Simple test that tests the basic usage of PEFT model through `from_pretrained`. This test tests if