Implement zero terminal SNR schedule option

modules/processing.py

@@ -863,6 +863,34 @@ def process_images_inner(p: StableDiffusionProcessing) -> Processed:
 
             if p.n_iter > 1:
                 shared.state.job = f"Batch {n+1} out of {p.n_iter}"
+                
+            def rescale_zero_terminal_snr_abar(alphas_cumprod):
+                alphas_bar_sqrt = alphas_cumprod.sqrt()
+
+                # Store old values.
+                alphas_bar_sqrt_0 = alphas_bar_sqrt[0].clone()
+                alphas_bar_sqrt_T = alphas_bar_sqrt[-1].clone()
+
+                # Shift so the last timestep is zero.
+                alphas_bar_sqrt -= (alphas_bar_sqrt_T)
+
+                # Scale so the first timestep is back to the old value.
+                alphas_bar_sqrt *= alphas_bar_sqrt_0 / (alphas_bar_sqrt_0 - alphas_bar_sqrt_T)
+
+                # Convert alphas_bar_sqrt to betas
+                alphas_bar = alphas_bar_sqrt**2  # Revert sqrt
+                alphas_bar[-1] = 4.8973451890853435e-08
+                return alphas_bar
+            
+            p.sd_model.alphas_cumprod = p.sd_model.alphas_cumprod_original.to(shared.device)
+
+            if opts.use_downcasted_alpha_bar:
+                p.extra_generation_params['Downcast alphas_cumprod'] = opts.use_downcasted_alpha_bar
+                p.sd_model.alphas_cumprod = p.sd_model.alphas_cumprod.half().to(shared.device)
+            if opts.sd_noise_schedule == "Zero Terminal SNR":
+                p.extra_generation_params['Noise Schedule'] = opts.sd_noise_schedule
+                print("rescaling noise schedule for zero snr")
+                p.sd_model.alphas_cumprod = rescale_zero_terminal_snr_abar(p.sd_model.alphas_cumprod).to(shared.device)
 
             with devices.without_autocast() if devices.unet_needs_upcast else devices.autocast():
                 samples_ddim = p.sample(conditioning=p.c, unconditional_conditioning=p.uc, seeds=p.seeds, subseeds=p.subseeds, subseed_strength=p.subseed_strength, prompts=p.prompts)