feat(nodes): add freeu support

Add support for FreeU. See:
- https://huggingface.co/docs/diffusers/main/en/using-diffusers/freeu
- https://github.com/ChenyangSi/FreeU

Implementation:
- `ModelPatcher.apply_freeu()` handles the enabling freeu (which is very simple with diffusers).
- `FreeUConfig` model added to hold the hyperparameters.
- `freeu_config` added as optional sub-field on `UNetField`.
- `FreeUInvocation` added, works like LoRA - chain it to add the FreeU config to the UNet
- No support for model-dependent presets, this will be a future workflow editor enhancement

Closes #4845

invokeai/app/invocations/baseinvocation.py

@@ -95,6 +95,10 @@ class FieldDescriptions:
     inclusive_low = "The inclusive low value"
     exclusive_high = "The exclusive high value"
     decimal_places = "The number of decimal places to round to"
+    freeu_s1 = 'Scaling factor for stage 1 to attenuate the contributions of the skip features. This is done to mitigate the "oversmoothing effect" in the enhanced denoising process.'
+    freeu_s2 = 'Scaling factor for stage 2 to attenuate the contributions of the skip features. This is done to mitigate the "oversmoothing effect" in the enhanced denoising process.'
+    freeu_b1 = "Scaling factor for stage 1 to amplify the contributions of backbone features."
+    freeu_b2 = "Scaling factor for stage 2 to amplify the contributions of backbone features."
 
 
 class Input(str, Enum):