Export ModelMixin so encoder from_pretrained can resolve

spectrogram_diffusion/__init__.py

@@ -1,13 +1,19 @@
 """Shim for this checkpoint's model_index.json.
 
 The Google weights name the note/context encoders as library
-`spectrogram_diffusion` (the original Magenta package). Current Diffusers
-keeps those classes under `diffusers.pipelines.deprecated`. This module
-makes `import spectrogram_diffusion` work inside the Inference Endpoint
-snapshot so `from_pretrained()` can load the encoders.
+`spectrogram_diffusion`. Diffusers' from_pretrained() does:
+
+    library = importlib.import_module("spectrogram_diffusion")
+    class_obj = getattr(library, "SpectrogramContEncoder")
+    class_candidates = {name: getattr(library, name, None) for name in LOADABLE_CLASSES}
+
+so this module must export both the encoder classes *and* ModelMixin
+(otherwise loading raises: no from_pretrained on the component).
 """
 from __future__ import annotations
 
+from diffusers import ModelMixin
+
 try:
     from diffusers.pipelines.deprecated.spectrogram_diffusion.continuous_encoder import (
         SpectrogramContEncoder,
@@ -23,4 +29,4 @@ except ImportError:  # older Diffusers still used the non-deprecated path
         SpectrogramNotesEncoder,
     )
 
-__all__ = ["SpectrogramContEncoder", "SpectrogramNotesEncoder"]
+__all__ = ["ModelMixin", "SpectrogramContEncoder", "SpectrogramNotesEncoder"]