By default the [`ControlNetUnionModel`] should be loaded with [`~ModelMixin.from_pretrained`].  
```py
from diffusers import StableDiffusionXLControlNetUnionPipeline, ControlNetUnionModel

controlnet = ControlNetUnionModel.from_pretrained("xinsir/controlnet-union-sdxl-1.0")
pipe = StableDiffusionXLControlNetUnionPipeline.from_pretrained("stabilityai/stable-diffusion-xl-base-1.0", controlnet=controlnet)
```