feat: add prediction argument

[daniandtheweb]

This PR adds an argument to manually select the prediction type. This allows the user to override the prediction type of the model on some experimental models that use non standard prediction types.

This argument aims to have a similar way to achieve what ComfyUI has with their ModelSamplingDiscrete node.

[stduhpf]

Maybe it would make sense to also add support for the recently added EDM_V_PRED that is used by CosXL?

[daniandtheweb]

I haven't been up to date with the recent development so I'm trying to catch up for now. Has it already been added to the repo right? I may be able to add support for it later.

[stduhpf]

Yes it has been added with #683

Also there's a distinction between Flux Flow and Sd3 (Discrete) Flow:
https://github.com/leejet/stable-diffusion.cpp/blob/master/stable-diffusion.cpp#L584C1-L606C1

[daniandtheweb]

I've added the new prediction methods. I'm unable to check them as I'm on a laptop right now but the logic is the same as with the others so it should work.

[daniandtheweb]

@stduhpf Feel free to push this PR on your bleedingedge fork or let me know if you think I could improve the code in any way (I'm still not sure if the description is clear enough).
I'd really love at least a comment from the main dev but unfortunately he doesn't seem to be very active around here.

[stduhpf]

I think I will re-create the bleedingedge branch from current master with the new c API (#728), I will try to include this PR when I do so.

[daniandtheweb]

Will you include the image preview PR? I'm building a webui (more like a command generator + gallery) and that would be very helpful. I've been thinking of adding a toggle to switch between a mode that works on the mainline branch and another that works on your fork (I still need to add all the new features from mainline but at least that's the plan).