Show error when loading safety_checker `from_flax` (#2187)

* Show error when loading safety_checker `from_flax`

* fix style

src/diffusers/pipelines/pipeline_utils.py

@@ -595,6 +595,14 @@ class DiffusionPipeline(ConfigMixin):
 
         init_dict = {k: v for k, v in init_dict.items() if load_module(k, v)}
 
+        # Special case: safety_checker must be loaded separately when using `from_flax`
+        if from_flax and "safety_checker" in init_dict and "safety_checker" not in passed_class_obj:
+            raise NotImplementedError(
+                "The safety checker cannot be automatically loaded when loading weights `from_flax`."
+                " Please, pass `safety_checker=None` to `from_pretrained`, and load the safety checker"
+                " separately if you need it."
+            )
+
         if len(unused_kwargs) > 0:
             logger.warning(
                 f"Keyword arguments {unused_kwargs} are not expected by {pipeline_class.__name__} and will be ignored."