[IR] Handle invalid output deserialization (#2223)

Handle deserializing a graph if an output that is not produced by any
nodes.

This is discovered when working on
microsoft/onnxruntime-genai#1416

Author: justinchuby

onnxscript/ir/serde.py

@@ -672,8 +672,23 @@ def _deserialize_graph(
         for node in proto.node
     ]
 
-    # Fill in values for graph outputs
-    outputs = [deserialize_value_info_proto(info, values[info.name]) for info in proto.output]
+    outputs = []
+    for info in proto.output:
+        # Fill in values for graph outputs
+        output_name = info.name
+        if output_name not in values:
+            # Handle (invalid) graph outputs that do not have any producers
+            logger.warning(
+                "Output '%s' is not produced by any node. The graph has an invalid output",
+                output_name,
+            )
+            value = _core.Value(name=output_name)
+        else:
+            # A valid, normal graph output
+            value = values[output_name]
+        # Fill in shape/type information
+        deserialize_value_info_proto(info, value)
+        outputs.append(value)
 
     # Exit the graph scope by popping the values for this scope from the stack
     scoped_values.pop()

onnxscript/ir/serde_test.py

@@ -290,6 +290,23 @@ def test_deserialize_graph_handles_unsorted_graph(self):
         self.assertEqual(deserialized_graph[0].op_type, "Op_1")
         self.assertEqual(deserialized_graph[1].op_type, "Op_0")
 
+    def test_deserialize_graph_handles_invalid_output(self):
+        # The graph has an output that is not connected to any node, and it does not
+        # have shape/type information.
+        graph_with_invalid_output = ir.Graph(
+            inputs=[],
+            outputs=[ir.Value(name="invalid_output")],
+            nodes=[],
+            name="graph_with_invalid_output",
+        )
+        graph_proto = serde.serialize_graph(graph_with_invalid_output)
+        deserialized_graph = serde.deserialize_graph(graph_proto)
+        self.assertEqual(len(deserialized_graph.outputs), 1)
+        self.assertEqual(deserialized_graph.outputs[0].name, "invalid_output")
+        self.assertEqual(deserialized_graph.outputs[0].type, None)
+        self.assertEqual(deserialized_graph.outputs[0].shape, None)
+        self.assertEqual(deserialized_graph.outputs[0].dtype, None)
+
 
 class QuantizationAnnotationTest(unittest.TestCase):
     """Test that quantization annotations are correctly serialized and deserialized."""