[Docs] add stable cascade unet doc. (#9066)

* add stable cascade unet doc.

* fix path

docs/source/en/_toctree.yml

@@ -237,6 +237,8 @@
       title: AutoencoderKL
     - local: api/models/asymmetricautoencoderkl
       title: AsymmetricAutoencoderKL
+    - local: api/models/stable_cascade_unet
+      title: StableCascadeUNet
     - local: api/models/autoencoder_tiny
       title: Tiny AutoEncoder
     - local: api/models/autoencoder_oobleck

docs/source/en/api/models/stable_cascade_unet.md

+<!--Copyright 2024 The HuggingFace Team. All rights reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with
+the License. You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on
+an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
+specific language governing permissions and limitations under the License.
+-->
+
+# StableCascadeUNet
+
+A UNet model from the [Stable Cascade pipeline](../pipelines/stable_cascade.md).
+
+## StableCascadeUNet
+
+[[autodoc]] models.unets.unet_stable_cascade.StableCascadeUNet