Change scheduler to DDPM

As it is a more robust scheduler in the Diffusers library, DDPM produces the best samples for Juggernaut XL v9 and should be in use by default please.

model_index.json

@@ -12,7 +12,7 @@
   ],
   "scheduler": [
     "diffusers",
-    "EulerDiscreteScheduler"
+    "DDPMScheduler"
   ],
   "text_encoder": [
     "transformers",