feat: add prediction argument

[daniandtheweb]

This PR adresses the issue https://github.com/leejet/stable-diffusion.cpp/issues/325 by adding an argument to force a specific prediction mode.

Some experimental models (such as EasyFluff and similar models) may introduce different prediction modes from what's expected (Stable Diffusion 1.5 uses EPS but in this case the mentioned model uses v-predict) and this change allows using them correctly by manually adjusting the prediction type.