fix unipc img2img denoising sample count

was wrongly using the inverse of the intended value. smaller denoising
strength should run fewer steps.

modules/models/diffusion/uni_pc/sampler.py

@@ -47,7 +47,7 @@ class UniPCSampler(object):
 
         # actual number of steps we'll run
         self.steps = max(
-                num_inference_steps - init_timestep,
+                init_timestep,
                 shared.opts.uni_pc_order+1,
             )