Modular backend - add FreeU (#6641)

## Summary

FreeU code from https://github.com/invoke-ai/InvokeAI/pull/6577.
Also fix issue with sometimes slightly different output.

## Related Issues / Discussions

#6606 

https://invokeai.notion.site/Modular-Stable-Diffusion-Backend-Design-Document-e8952daab5d5472faecdc4a72d377b0d

## QA Instructions

Run with and without set `USE_MODULAR_DENOISE` environment.

## Merge Plan

Nope.
If you think that there should be some kind of tests - feel free to add.

## Checklist

- [x] _The PR has a short but descriptive title, suitable for a
changelog_
- [ ] _Tests added / updated (if applicable)_
- [ ] _Documentation added / updated (if applicable)_

invokeai/app/invocations/denoise_latents.py

@@ -58,6 +58,7 @@ from invokeai.backend.stable_diffusion.diffusion.conditioning_data import (
 from invokeai.backend.stable_diffusion.diffusion.custom_atttention import CustomAttnProcessor2_0
 from invokeai.backend.stable_diffusion.diffusion_backend import StableDiffusionBackend
 from invokeai.backend.stable_diffusion.extension_callback_type import ExtensionCallbackType
+from invokeai.backend.stable_diffusion.extensions.freeu import FreeUExt
 from invokeai.backend.stable_diffusion.extensions.preview import PreviewExt
 from invokeai.backend.stable_diffusion.extensions.rescale_cfg import RescaleCFGExt
 from invokeai.backend.stable_diffusion.extensions_manager import ExtensionsManager
@@ -795,18 +796,22 @@ class DenoiseLatentsInvocation(BaseInvocation):
         if self.cfg_rescale_multiplier > 0:
             ext_manager.add_extension(RescaleCFGExt(self.cfg_rescale_multiplier))
 
+        ### freeu
+        if self.unet.freeu_config:
+            ext_manager.add_extension(FreeUExt(self.unet.freeu_config))
+
         # ext: t2i/ip adapter
         ext_manager.run_callback(ExtensionCallbackType.SETUP, denoise_ctx)
 
         unet_info = context.models.load(self.unet.unet)
         assert isinstance(unet_info.model, UNet2DConditionModel)
         with (
-            unet_info.model_on_device() as (model_state_dict, unet),
+            unet_info.model_on_device() as (cached_weights, unet),
             ModelPatcher.patch_unet_attention_processor(unet, denoise_ctx.inputs.attention_processor_cls),
             # ext: controlnet
             ext_manager.patch_extensions(unet),
             # ext: freeu, seamless, ip adapter, lora
-            ext_manager.patch_unet(model_state_dict, unet),
+            ext_manager.patch_unet(unet, cached_weights),
         ):
             sd_backend = StableDiffusionBackend(unet, scheduler)
             denoise_ctx.unet = unet

invokeai/backend/stable_diffusion/diffusion_backend.py

@@ -100,8 +100,10 @@ class StableDiffusionBackend:
         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 this `torch.lerp(...)` line is logically equivalent to the current CFG line, it seems to result
+        # in slightly different outputs. It is suspected that this is caused by small precision differences.
+        # 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

invokeai/backend/stable_diffusion/extensions/base.py

@@ -2,7 +2,7 @@ from __future__ import annotations
 
 from contextlib import contextmanager
 from dataclasses import dataclass
-from typing import TYPE_CHECKING, Callable, Dict, List
+from typing import TYPE_CHECKING, Callable, Dict, List, Optional
 
 import torch
 from diffusers import UNet2DConditionModel
@@ -56,5 +56,5 @@ class ExtensionBase:
         yield None
 
     @contextmanager
-    def patch_unet(self, state_dict: Dict[str, torch.Tensor], unet: UNet2DConditionModel):
+    def patch_unet(self, unet: UNet2DConditionModel, cached_weights: Optional[Dict[str, torch.Tensor]] = None):
         yield None

invokeai/backend/stable_diffusion/extensions/freeu.py

@@ -0,0 +1,35 @@
+from __future__ import annotations
+
+from contextlib import contextmanager
+from typing import TYPE_CHECKING, Dict, Optional
+
+import torch
+from diffusers import UNet2DConditionModel
+
+from invokeai.backend.stable_diffusion.extensions.base import ExtensionBase
+
+if TYPE_CHECKING:
+    from invokeai.app.shared.models import FreeUConfig
+
+
+class FreeUExt(ExtensionBase):
+    def __init__(
+        self,
+        freeu_config: FreeUConfig,
+    ):
+        super().__init__()
+        self._freeu_config = freeu_config
+
+    @contextmanager
+    def patch_unet(self, unet: UNet2DConditionModel, cached_weights: Optional[Dict[str, torch.Tensor]] = None):
+        unet.enable_freeu(
+            b1=self._freeu_config.b1,
+            b2=self._freeu_config.b2,
+            s1=self._freeu_config.s1,
+            s2=self._freeu_config.s2,
+        )
+
+        try:
+            yield
+        finally:
+            unet.disable_freeu()

invokeai/backend/stable_diffusion/extensions_manager.py

@@ -63,9 +63,13 @@ class ExtensionsManager:
             yield None
 
     @contextmanager
-    def patch_unet(self, state_dict: Dict[str, torch.Tensor], unet: UNet2DConditionModel):
+    def patch_unet(self, unet: UNet2DConditionModel, cached_weights: Optional[Dict[str, torch.Tensor]] = None):
         if self._is_canceled and self._is_canceled():
             raise CanceledException
 
-        # TODO: create logic in PR with extension which uses it
+        # TODO: create weight patch logic in PR with extension which uses it
+        with ExitStack() as exit_stack:
+            for ext in self._extensions:
+                exit_stack.enter_context(ext.patch_unet(unet, cached_weights))
+
             yield None