Fix uni_pc returning noisy image when steps <= 3

comfy/extra_samplers/uni_pc.py

@@ -881,7 +881,7 @@ def sample_unipc(model, noise, image, sigmas, sampling_function, max_denoise, ex
             model_kwargs=extra_args,
         )
 
-        order = min(3, len(timesteps) - 1)
+        order = min(3, len(timesteps) - 2)
         uni_pc = UniPC(model_fn, ns, predict_x0=True, thresholding=False, noise_mask=noise_mask, masked_image=image, noise=noise, variant=variant)
         x = uni_pc.sample(img, timesteps=timesteps, skip_type="time_uniform", method="multistep", order=order, lower_order_final=True, callback=callback, disable_pbar=disable)
         x /= ns.marginal_alpha(timesteps[-1])