Keep inputs of placeholder with default when removing unused nodes (#1510)

Signed-off-by: Tom Wildenhain <[email protected]>

Author: TomWildenhain-Microsoft

tf2onnx/graph.py

@@ -1505,17 +1505,17 @@ def extract_sub_graph_nodes(self, outputs_name, input_checker=None, remove_unuse
             a list of nodes
         """
         res_set = set()
-        if not outputs_name:
-            return list(res_set)
 
-        for output in outputs_name:
+        outputs_to_keep = list(outputs_name)
+        if not remove_unused_inputs:
+            # add placeholder nodes even if they are not connected to outputs.
+            # placeholder nodes with defaults can have inputs themselves
+            outputs_to_keep += [inp.output[0] for inp in self.inputs]
+
+        for output in outputs_to_keep:
             node = self.get_node_by_output(output, search_in_parent_graphs=False)
             res_set = res_set.union(self._extract_sub_graph_nodes(node, input_checker))
 
-        if not remove_unused_inputs:
-            # add back placeholder nodes if they are not connected to outputs.
-            res_set = res_set.union(self.inputs)
-
         return list(res_set)
 
     def delete_unused_nodes(self, outputs_name):