Fix slightly different output with old backend

Author: StAlKeR7779

invokeai/backend/stable_diffusion/diffusion_backend.py

@@ -100,8 +100,10 @@ def apply_cfg(ctx: DenoiseContext) -> torch.Tensor:
         if isinstance(guidance_scale, list):
             guidance_scale = guidance_scale[ctx.step_index]
 
-        return torch.lerp(ctx.negative_noise_pred, ctx.positive_noise_pred, guidance_scale)
-        # return ctx.negative_noise_pred + guidance_scale * (ctx.positive_noise_pred - ctx.negative_noise_pred)
+        # Note: Although logically it same, it seams that precision errors differs.
+        # This sometimes results in slightly different output.
+        # return torch.lerp(ctx.negative_noise_pred, ctx.positive_noise_pred, guidance_scale)
+        return ctx.negative_noise_pred + guidance_scale * (ctx.positive_noise_pred - ctx.negative_noise_pred)
 
     def run_unet(self, ctx: DenoiseContext, ext_manager: ExtensionsManager, conditioning_mode: ConditioningMode):
         sample = ctx.latent_model_input