ase can be found here: 
Stable Diffusion v2: Stability-AI/stablediffusion
Available Checkpoints are:
stabilityai/stable-diffusion-x4-upscaler (x4 resolution resolution): stable-diffusion-x4-upscaler

class diffusers.StableDiffusionUpscalePipeline

<
source
>
(
vae: AutoencoderKL
text_encoder: CLIPTextModel
tokenizer: CLIPTokenizer
unet: UNet2DConditionModel
low_res_scheduler: DDPMScheduler
scheduler: KarrasDiffusionSchedulers
safety_checker: ty