Enable `requires_grad` on input embedding to train on top of frozen layers (#21598)

* v1

* make fixup

* add more methods

src/transformers/modeling_utils.py

@@ -1148,6 +1148,23 @@ class PreTrainedModel(nn.Module, ModuleUtilsMixin, GenerationMixin, PushToHubMix
             return False
         return True
 
+    def enable_input_require_grads(self):
+        """
+        Enables the gradients for the input embeddings. This is useful for fine-tuning adapter weights while keeping
+        the model weights fixed.
+        """
+
+        def make_inputs_require_grads(module, input, output):
+            output.requires_grad_(True)
+
+        self._require_grads_hook = self.get_input_embeddings().register_forward_hook(make_inputs_require_grads)
+
+    def disable_input_require_grads(self):
+        """
+        Removes the `_require_grads_hook`.
+        """
+        self._require_grads_hook.remove()
+
     def get_input_embeddings(self) -> nn.Module:
         """
         Returns the model's input embeddings.