Return Flax scheduler state (#601)

* Optionally return state in from_config.

Useful for Flax schedulers.

* has_state is now a property, make check more strict.

I don't check the class is `SchedulerMixin` to prevent circular
dependencies. It should be enough that the class name starts with "Flax"
the object declares it "has_state" and the "create_state" exists too.

* Use state in pipeline from_pretrained.

* Make style

src/diffusers/configuration_utils.py

@@ -160,12 +160,19 @@ class ConfigMixin:
         if "dtype" in unused_kwargs:
             init_dict["dtype"] = unused_kwargs.pop("dtype")
 
+        # Return model and optionally state and/or unused_kwargs
         model = cls(**init_dict)
+        return_tuple = (model,)
+
+        # Flax schedulers have a state, so return it.
+        if cls.__name__.startswith("Flax") and hasattr(model, "create_state") and getattr(model, "has_state", False):
+            state = model.create_state()
+            return_tuple += (state,)
 
         if return_unused_kwargs:
-            return model, unused_kwargs
+            return return_tuple + (unused_kwargs,)
         else:
-            return model
+            return return_tuple if len(return_tuple) > 1 else model
 
     @classmethod
     def get_config_dict(

src/diffusers/pipeline_flax_utils.py

@@ -437,8 +437,8 @@ class FlaxDiffusionPipeline(ConfigMixin):
                         loaded_sub_model, loaded_params = load_method(loadable_folder, _do_init=False)
                     params[name] = loaded_params
                 elif issubclass(class_obj, SchedulerMixin):
-                    loaded_sub_model = load_method(loadable_folder)
-                    params[name] = loaded_sub_model.create_state()
+                    loaded_sub_model, scheduler_state = load_method(loadable_folder)
+                    params[name] = scheduler_state
                 else:
                     loaded_sub_model = load_method(loadable_folder)
 

src/diffusers/schedulers/scheduling_ddim_flax.py

@@ -105,6 +105,10 @@ class FlaxDDIMScheduler(SchedulerMixin, ConfigMixin):
             stable diffusion.
     """
 
+    @property
+    def has_state(self):
+        return True
+
     @register_to_config
     def __init__(
         self,

src/diffusers/schedulers/scheduling_pndm_flax.py

@@ -113,6 +113,10 @@ class FlaxPNDMScheduler(SchedulerMixin, ConfigMixin):
             stable diffusion.
     """
 
+    @property
+    def has_state(self):
+        return True
+
     @register_to_config
     def __init__(
         self,