Add preview to SubGraphOutputNode

com.unity.shadergraph/Editor/Data/SubGraph/SubGraphOutputNode.cs

             name = "SubGraphOutputs";
         }
 
+        public override bool hasPreview
+        {
+            get { return true; }
+        }
+
         public virtual int AddSlot()
         {
             var index = this.GetInputSlots<ISlot>().Count() + 1;