fix une2td ignoring class_labels (#5401)

* fix une2td ignoring class_labels

* unet2.py error message updated

* style and quality changes

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Co-authored-by: Dhruv Nair <dhruv.nair@gmail.com>

src/diffusers/models/unet_2d.py

@@ -291,6 +291,8 @@ class UNet2DModel(ModelMixin, ConfigMixin):
 
             class_emb = self.class_embedding(class_labels).to(dtype=self.dtype)
             emb = emb + class_emb
+        elif self.class_embedding is None and class_labels is not None:
+            raise ValueError("class_embedding needs to be initialized in order to use class conditioning")
 
         # 2. pre-process
         skip_sample = sample