[Transform] handle 1-node graphs correctly in SortGraph

src/finn/transformation/general.py

@@ -189,6 +189,9 @@ class SortGraph(Transformation):
     # Probably this is faster than copying initializers and more robust in general
 
     def apply(self, model):
+        if len(model.graph.node) == 1:
+            # single-node graph, nothing to sort
+            return (model, False)
         # Gather graph structure
         graph_dependencies = {}
         node_list = [
@@ -214,7 +217,7 @@ class SortGraph(Transformation):
         for new_idx, sorted_idx in enumerate(sorted_node_indexes):
             model.graph.node.insert(new_idx, node_list[sorted_idx])
 
-        return model, False
+        return (model, False)
 
 
 class ConvertSubToAdd(Transformation):