add cheap VAE approximation coeffs for SDXL

modules/sd_vae_approx.py

@@ -64,12 +64,22 @@ def model():
 def cheap_approximation(sample):
     # https://discuss.huggingface.co/t/decoding-latents-to-rgb-without-upscaling/23204/2
 
-    coefs = torch.tensor([
+    if shared.sd_model.is_sdxl:
+        coeffs = [
+            [ 0.3448,  0.4168,  0.4395],
+            [-0.1953, -0.0290,  0.0250],
+            [ 0.1074,  0.0886, -0.0163],
+            [-0.3730, -0.2499, -0.2088],
+        ]
+    else:
+        coeffs = [
             [ 0.298,  0.207,  0.208],
             [ 0.187,  0.286,  0.173],
             [-0.158,  0.189,  0.264],
             [-0.184, -0.271, -0.473],
-    ]).to(sample.device)
+        ]
+
+    coefs = torch.tensor(coeffs).to(sample.device)
 
     x_sample = torch.einsum("lxy,lr -> rxy", sample, coefs)