Fix VAE scale factor for Stable Diffusion XL (#79)

Fix vae scale factor for Stable Diffusion XL

Colors are much better now.

See apple/ml-stable-diffusion@ce8ee78

Author: pcuenca

Diffusion.xcodeproj/project.xcworkspace/xcshareddata/swiftpm/Package.resolved

@@ -15,7 +15,7 @@
       "location" : "https://github.com/apple/ml-stable-diffusion",
       "state" : {
         "branch" : "main",
-        "revision" : "8cf34376f9faf87fc6fe63159e5fae6cbbb71de6"
+        "revision" : "ce8ee78e28613d8a2e4c8b56932b236cb57e7e20"
       }
     },
     {

Diffusion/Common/Pipeline/Pipeline.swift

@@ -43,6 +43,13 @@ class Pipeline {
     let pipeline: StableDiffusionPipelineProtocol
     let maxSeed: UInt32
 
+    var isXL: Bool {
+        if #available(macOS 14.0, iOS 17.0, *) {
+            return (pipeline as? StableDiffusionXLPipeline) != nil
+        }
+        return false
+    }
+
     var progress: StableDiffusionProgress? = nil {
         didSet {
             progressPublisher.value = progress
@@ -81,6 +88,10 @@ class Pipeline {
         config.disableSafety = disableSafety
         config.schedulerType = scheduler.asStableDiffusionScheduler()
         config.useDenoisedIntermediates = true
+        if isXL {
+            config.encoderScaleFactor = 0.13025
+            config.decoderScaleFactor = 0.13025
+        }
 
         // Evenly distribute previews based on inference steps
         let previewIndices = previewIndices(stepCount, previewCount)