StableDiffusionLatentUpscalePipeline - positive/negative prompt embeds support (#8947)

* make latent upscaler accept prompt embeds

---------

Co-authored-by: Dhruv Nair <[email protected]>
Co-authored-by: Sayak Paul <[email protected]>
Co-authored-by: YiYi Xu <[email protected]>

Author: 4 people