instruct pix2pix pipeline: remove sigma scaling when computing classifier free guidance (#7006)

remove sigma scaling when computing cfg
Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>

src/diffusers/pipelines/stable_diffusion/pipeline_stable_diffusion_instruct_pix2pix.py

@@ -323,8 +323,6 @@ class StableDiffusionInstructPix2PixPipeline(
             batch_size = prompt_embeds.shape[0]
 
         device = self._execution_device
-        # check if scheduler is in sigmas space
-        scheduler_is_in_sigma_space = hasattr(self.scheduler, "sigmas")
 
         # 2. Encode input prompt
         prompt_embeds = self._encode_prompt(
@@ -411,15 +409,6 @@ class StableDiffusionInstructPix2PixPipeline(
                     return_dict=False,
                 )[0]
 
-                # Hack:
-                # For karras style schedulers the model does classifer free guidance using the
-                # predicted_original_sample instead of the noise_pred. So we need to compute the
-                # predicted_original_sample here if we are using a karras style scheduler.
-                if scheduler_is_in_sigma_space:
-                    step_index = (self.scheduler.timesteps == t).nonzero()[0].item()
-                    sigma = self.scheduler.sigmas[step_index]
-                    noise_pred = latent_model_input - sigma * noise_pred
-
                 # perform guidance
                 if self.do_classifier_free_guidance:
                     noise_pred_text, noise_pred_image, noise_pred_uncond = noise_pred.chunk(3)
@@ -429,15 +418,6 @@ class StableDiffusionInstructPix2PixPipeline(
                         + self.image_guidance_scale * (noise_pred_image - noise_pred_uncond)
                     )
 
-                # Hack:
-                # For karras style schedulers the model does classifer free guidance using the
-                # predicted_original_sample instead of the noise_pred. But the scheduler.step function
-                # expects the noise_pred and computes the predicted_original_sample internally. So we
-                # need to overwrite the noise_pred here such that the value of the computed
-                # predicted_original_sample is correct.
-                if scheduler_is_in_sigma_space:
-                    noise_pred = (noise_pred - latents) / (-sigma)
-
                 # compute the previous noisy sample x_t -> x_t-1
                 latents = self.scheduler.step(noise_pred, t, latents, **extra_step_kwargs, return_dict=False)[0]
 

src/diffusers/pipelines/stable_diffusion_xl/pipeline_stable_diffusion_xl_instruct_pix2pix.py

@@ -774,8 +774,6 @@ class StableDiffusionXLInstructPix2PixPipeline(
         # of the Imagen paper: https://arxiv.org/pdf/2205.11487.pdf . `guidance_scale = 1`
         # corresponds to doing no classifier free guidance.
         do_classifier_free_guidance = guidance_scale > 1.0 and image_guidance_scale >= 1.0
-        # check if scheduler is in sigmas space
-        scheduler_is_in_sigma_space = hasattr(self.scheduler, "sigmas")
 
         # 3. Encode input prompt
         text_encoder_lora_scale = (
@@ -906,15 +904,6 @@ class StableDiffusionXLInstructPix2PixPipeline(
                     return_dict=False,
                 )[0]
 
-                # Hack:
-                # For karras style schedulers the model does classifer free guidance using the
-                # predicted_original_sample instead of the noise_pred. So we need to compute the
-                # predicted_original_sample here if we are using a karras style scheduler.
-                if scheduler_is_in_sigma_space:
-                    step_index = (self.scheduler.timesteps == t).nonzero()[0].item()
-                    sigma = self.scheduler.sigmas[step_index]
-                    noise_pred = latent_model_input - sigma * noise_pred
-
                 # perform guidance
                 if do_classifier_free_guidance:
                     noise_pred_text, noise_pred_image, noise_pred_uncond = noise_pred.chunk(3)
@@ -928,15 +917,6 @@ class StableDiffusionXLInstructPix2PixPipeline(
                     # Based on 3.4. in https://arxiv.org/pdf/2305.08891.pdf
                     noise_pred = rescale_noise_cfg(noise_pred, noise_pred_text, guidance_rescale=guidance_rescale)
 
-                # Hack:
-                # For karras style schedulers the model does classifer free guidance using the
-                # predicted_original_sample instead of the noise_pred. But the scheduler.step function
-                # expects the noise_pred and computes the predicted_original_sample internally. So we
-                # need to overwrite the noise_pred here such that the value of the computed
-                # predicted_original_sample is correct.
-                if scheduler_is_in_sigma_space:
-                    noise_pred = (noise_pred - latents) / (-sigma)
-
                 # compute the previous noisy sample x_t -> x_t-1
                 latents = self.scheduler.step(noise_pred, t, latents, **extra_step_kwargs, return_dict=False)[0]