[Lora] Speed up lora loading (#4994)

* speed up lora loading

* Apply suggestions from code review

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* up

* Fix more

* Correct more

* Apply suggestions from code review

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src/diffusers/models/modeling_utils.py

@@ -128,6 +128,31 @@ def load_state_dict(checkpoint_file: Union[str, os.PathLike], variant: Optional[
             )
 
 
+def load_model_dict_into_meta(model, state_dict, device=None, dtype=None, model_name_or_path=None):
+    device = device or torch.device("cpu")
+    dtype = dtype or torch.float32
+
+    unexpected_keys = []
+    empty_state_dict = model.state_dict()
+    for param_name, param in state_dict.items():
+        if param_name not in empty_state_dict:
+            unexpected_keys.append(param_name)
+            continue
+
+        if empty_state_dict[param_name].shape != param.shape:
+            model_name_or_path_str = f"{model_name_or_path} " if model_name_or_path is not None else ""
+            raise ValueError(
+                f"Cannot load {model_name_or_path_str}because {param_name} expected shape {empty_state_dict[param_name]}, but got {param.shape}. If you want to instead overwrite randomly initialized weights, please make sure to pass both `low_cpu_mem_usage=False` and `ignore_mismatched_sizes=True`. For more information, see also: https://github.com/huggingface/diffusers/issues/1619#issuecomment-1345604389 as an example."
+            )
+
+        accepts_dtype = "dtype" in set(inspect.signature(set_module_tensor_to_device).parameters.keys())
+        if accepts_dtype:
+            set_module_tensor_to_device(model, param_name, device, value=param, dtype=dtype)
+        else:
+            set_module_tensor_to_device(model, param_name, device, value=param)
+    return unexpected_keys
+
+
 def _load_state_dict_into_model(model_to_load, state_dict):
     # Convert old format to new format if needed from a PyTorch state_dict
     # copy state_dict so _load_from_state_dict can modify it
@@ -624,29 +649,14 @@ class ModelMixin(torch.nn.Module, PushToHubMixin):
                             " `low_cpu_mem_usage=False` and `device_map=None` if you want to randomly initialize"
                             " those weights or else make sure your checkpoint file is correct."
                         )
-                    unexpected_keys = []
-
-                    empty_state_dict = model.state_dict()
-                    for param_name, param in state_dict.items():
-                        accepts_dtype = "dtype" in set(
-                            inspect.signature(set_module_tensor_to_device).parameters.keys()
-                        )
 
-                        if param_name not in empty_state_dict:
-                            unexpected_keys.append(param_name)
-                            continue
-
-                        if empty_state_dict[param_name].shape != param.shape:
-                            raise ValueError(
-                                f"Cannot load {pretrained_model_name_or_path} because {param_name} expected shape {empty_state_dict[param_name]}, but got {param.shape}. If you want to instead overwrite randomly initialized weights, please make sure to pass both `low_cpu_mem_usage=False` and `ignore_mismatched_sizes=True`. For more information, see also: https://github.com/huggingface/diffusers/issues/1619#issuecomment-1345604389 as an example."
-                            )
-
-                        if accepts_dtype:
-                            set_module_tensor_to_device(
-                                model, param_name, param_device, value=param, dtype=torch_dtype
+                    unexpected_keys = load_model_dict_into_meta(
+                        model,
+                        state_dict,
+                        device=param_device,
+                        dtype=torch_dtype,
+                        model_name_or_path=pretrained_model_name_or_path,
                     )
-                        else:
-                            set_module_tensor_to_device(model, param_name, param_device, value=param)
 
                     if cls._keys_to_ignore_on_load_unexpected is not None:
                         for pat in cls._keys_to_ignore_on_load_unexpected: