Round image dimensions to the specific multiple required by each
DiT model, which range from 32 (certain Wan models) to 1 (Chroma
Radiance), with most requiring multiples of 8 or 16. Unet models
keep being rounded to multiples of 64.
Current sd.cpp rounds the sizes internally; but it always rounds
up, so we still need to round on our side to apply image size
restrictions, and to trigger VAE tiling correctly.
Also, remove a legacy test that could abort a generation with
unsupported image sizes: it'd never run, because it was applied
after the image side adjustements.
Wagner Bruna
19588f18ea