improve hires fix efficiency; fix: better SDXL hires fix default implementation; fix: cascade bug

[tazlin]

• fix: resolve bug with cascade steps not applying correctly
• fix: better SDXL hires + calc steps more effectively
  • Hires fix now (generally speaking) does the following by default:
    • Uses fewer steps when 'hires_fix_denoise_strength' is around 0.65 and the target resolution is not too far from the baseline resolution.
      • This is consistent with a very low drop in quality for 'average' settings while on average using far fewer (1/3-1/2) of the hires res second pass steps ('upscale steps').
    • Uses slightly fewer or about the same steps when 'hires_fix_denoise_strength' is around 0.65 and the target resolution is much higher (in terms of width*hieght) from the baseline resolution.
      • This is consistent with maintaining the level of quality which is currently status quo with these types of generations. The further the target resolution is from the baseline resolution ultimately requires more upscale steps. I suspect this is the key reason @jug-dev originally implemented hires fix in the original way (always using the same requested steps and upscale steps).
    • Uses more steps when hires_fix_denoise_strength is closer to 1.0
      • This is to account for the fact that denoise this high requires many more steps to "resolve" into a coherent image.
    • As hires_fix_denoise_strength approaches 0, uses far fewer steps.
      • Low values tend to converge onto a final result very quickly, rendering many steps moot.
  • Accordingly, these new dynamics are now in play:
    • Upscale steps can never be lower than 6 unless the requested steps are less than 6, in which case upscale steps cannot be lower than that amount.
    • Upscale steps can never exceed the number of requested steps for the job.
  • SDXL hires fix now always picks the closest officially recommended SDXL resolution (by aspect ratio) for the first pass. The second pass will remain the resolution requested by the user.
    • See IDEAL_SDXL_RESOLUTIONS for the resolutions that can be chosen.
• Several minor tweaks to the pytests for better dev-friendliness and to minimize time lost to long-running test runs that are doomed to fail.

[db0]

wooo!