[Optimizer] Avoid accessing None value in _process_constant_node (#2513)

Signed-off-by: Christoph Berganski <christoph.berganski@gmail.com>

Author: iksnagreb

onnxscript/optimizer/_constant_folding.py

@@ -71,6 +71,12 @@ def _process_constant_node(node: ir.Node) -> None:
     if attr_value is None or not isinstance(attr_value, ir.Attr):
         return
 
+    # Even if this is an attribute, the value property might not be set, which
+    # happens e.g. in case of attribute references, i.e., ref_attr_name is set
+    if attr_value.value is None:
+        # For now reject this to prevent TypeError from accessing Nones below
+        return
+
     const_value: ir.TensorProtocol
     if attr_name in {"value_float", "value_floats"}:
         const_value = ir.Tensor(

onnxscript/optimizer/_constant_folding_test.py

@@ -597,6 +597,22 @@ def test_multi_graph_identity_output_preserves_output_name(self):
         )
         self.assertEqual([input.name for input in optimized.graph.inputs], ["x"])
 
+    # This should not be constant-foldable as the constant references an
+    # attribute and thus the shape cannot be resolved. At the same time it
+    # should not fail due to the attribute value being None in
+    # _process_constant_node
+    def test_attribute_reference(self):
+        model = """
+            <ir_version: 7, opset_import: ["" : 17]>
+            agraph () => (int64[N] z) {
+                x = Constant <value_ints: ints = @attr> ()
+                z = Shape (x)
+            }
+        """
+
+        optimized = self._fold(model)
+        self.assertEqual(len(optimized.graph), 2)
+
 
 if __name__ == "__main__":
     unittest.main()