Generate placeholder graph inputs for None optional inputs

When _split_optional_inputs encounters a None entry, it now creates an
untyped ir.Value placeholder (named input_N) and adds it to graph_inputs.
This ensures the function signature declares all formal parameters,
including absent optional ones, while the trace function still receives
None so it can branch with 'if x is None:'.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Signed-off-by: G Ramalingam <grama@microsoft.com>

Author: gramalingam

onnxscript/_internal/builder.py

@@ -194,25 +194,34 @@ def _split_optional_inputs(
 ) -> tuple[list[ir.Value | None], list[ir.Value]]:
     """Split an input list into trace args and graph inputs.
 
+    For each ``None`` entry, a placeholder :class:`ir.Value` with a generated
+    name (``input_0``, ``input_1``, …) is created and added to
+    *graph_inputs* so that the function/graph signature declares the formal
+    parameter.  The corresponding *trace_args* entry remains ``None`` so that
+    the trace function can branch with ``if x is None:``.
+
     Returns:
         A tuple of (trace_args, graph_inputs) where trace_args preserves
-        ``None`` holes and graph_inputs contains only non-``None`` values.
+        ``None`` holes and graph_inputs includes placeholders for absent
+        optional inputs.
 
     Raises:
         ValueError: If any non-None input already has a producer or is
             already attached to a graph.
     """
     trace_args: list[ir.Value | None] = list(inputs)
     graph_inputs: list[ir.Value] = []
-    for v in trace_args:
+    for i, v in enumerate(trace_args):
         if v is None:
-            continue
-        if v.producer() is not None:
-            raise ValueError(
-                f"Input {v.name!r} already has a producer node. "
-                f"Pass freshly created ir.Value objects."
-            )
-        graph_inputs.append(v)
+            # Placeholder: declared in function signature but unused in body.
+            graph_inputs.append(ir.Value(name=f"input_{i}"))
+        else:
+            if v.producer() is not None:
+                raise ValueError(
+                    f"Input {v.name!r} already has a producer node. "
+                    f"Pass freshly created ir.Value objects."
+                )
+            graph_inputs.append(v)
     return trace_args, graph_inputs
 
 

onnxscript/_internal/builder_test.py

@@ -1750,7 +1750,7 @@ def test_build_function_lifts_initializers(self):
         self.assertGreater(len(const_nodes), 0)
 
     def test_build_function_optional_inputs_none(self):
-        """build_function passes None for absent optional inputs."""
+        """build_function passes None for absent optional inputs but declares them."""
 
         def body(op, x, y, z):
             self.assertIsNotNone(x)
@@ -1764,10 +1764,12 @@ def body(op, x, y, z):
             domain="com.test",
             name="OptionalInputs",
         )
-        # Graph should have only 2 inputs (x and z), not 3
-        self.assertEqual(len(fn.graph.inputs), 2)
+        # Graph has 3 inputs: x, a placeholder for the absent y, and z
+        self.assertEqual(len(fn.graph.inputs), 3)
         self.assertEqual(fn.graph.inputs[0].name, "x")
-        self.assertEqual(fn.graph.inputs[1].name, "z")
+        self.assertEqual(fn.graph.inputs[1].name, "input_1")  # placeholder
+        self.assertIsNone(fn.graph.inputs[1].type)  # untyped
+        self.assertEqual(fn.graph.inputs[2].name, "z")
 
     def test_build_function_trace_appends_outputs(self):
         """build_function supports trace functions that append outputs directly."""