Standardize on `init_image` for img2img, inpaint, and others

[fboulnois]

What API design would you like to have changed or added to the library? Why?

img2img currently uses the init_image parameter, whereas inpaint uses the image parameter.

What use case would this enable or better enable? Can you give us a code example?

I have some code that dynamically switches between StableDiffusionPipeline, StableDiffusionImg2ImgPipeline, and StableDiffusionInpaintPipeline based on whether an image and mask are provided, and standardizing on init_image would simplify the pipeline call (instead of checking beforehand / supplying unused parameters):

pipeline(prompt, init_image=image, ...)

[patrickvonplaten]

fair point! Would be ok with just using image=... everywhere. What do you think @pcuenca , @patil-suraj @anton-l ?

[pcuenca]

I agree, it's a good point and I'd rather standardize on image rather than init_image. @fboulnois would you like to open a PR with that? It's totally cool if you don't have time though :)