[Safetensors] Make sure metadata is saved (#2506)

* [Safetensors] Make sure metadata is saved

* make style

src/diffusers/models/modeling_utils.py

@@ -291,9 +291,6 @@ class ModelMixin(torch.nn.Module):
             logger.error(f"Provided path ({save_directory}) should be a directory, not a file")
             return
 
-        if save_function is None:
-            save_function = safetensors.torch.save_file if safe_serialization else torch.save
-
         os.makedirs(save_directory, exist_ok=True)
 
         model_to_save = self
@@ -310,7 +307,12 @@ class ModelMixin(torch.nn.Module):
         weights_name = _add_variant(weights_name, variant)
 
         # Save the model
-        save_function(state_dict, os.path.join(save_directory, weights_name))
+        if safe_serialization:
+            safetensors.torch.save_file(
+                state_dict, os.path.join(save_directory, weights_name), metadata={"format": "pt"}
+            )
+        else:
+            torch.save(state_dict, os.path.join(save_directory, weights_name))
 
         logger.info(f"Model weights saved in {os.path.join(save_directory, weights_name)}")