max_items_to_save for `guidance_eval()`

[claforte]

• currently limited to zero123_guidance and stable_diffusion_guidance
• limits number of batch items to evaluate guidance for debugging purposes
  • defaults to 4
  • higher values (e.g. 12) resulted in very large images that were harder to analyze, took longer to evaluate
• also slightly improve hyperparams for anya phase 1
• TODO: avoid duplicating code...
• part of guidance_eval logic should probably be moved in base class