feat: store all initializers in root graph, not just cached constants

Extend the root-graph invariant to the initializer() method itself,
not just the cached literal path in _get_or_create_constant(). Now
every call to builder.initializer() registers the value in
self._root._graph, ensuring that direct initializer creation from
sub-builders also goes to the root graph.

This maintains a clean invariant: all initializers live in the root
graph, regardless of which builder created them.

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

Author: gramalingam

onnxscript/_internal/builder.py

@@ -318,7 +318,14 @@ def graph(self) -> ir.Graph:
     def initializer(
         self, tensor: ir.TensorProtocol, name: str | None = None, *, qualify: bool = True
     ) -> ir.Value:
-        """Register a tensor as a graph initializer, returning the corresponding ir.Value."""
+        """Register a tensor as a graph initializer in the **root** graph.
+
+        All initializers are stored in the root graph so that inner scopes
+        (subgraphs) can reference them via ONNX's outer-scope visibility
+        rules.  For function bodies (which cannot have initializers), apply
+        :func:`lift_initializers_to_constants` before wrapping in
+        :class:`ir.Function`.
+        """
         if name is None:
             name = tensor.name
         if qualify:
@@ -327,7 +334,7 @@ def initializer(
         value = ir.Value(
             name=name, shape=shape, type=ir.TensorType(tensor.dtype), const_value=tensor
         )
-        self._graph.register_initializer(value)
+        self._root._graph.register_initializer(value)
         return value
 
     def input(

onnxscript/_internal/builder_test.py

@@ -1504,6 +1504,29 @@ def body2(op, x):
         const_names = [n for n in root_graph.initializers if "const_2.0" in n]
         self.assertEqual(len(const_names), 1, f"Expected 1 shared initializer: {const_names}")
 
+    def test_direct_initializer_in_subgraph_goes_to_root(self):
+        """builder.initializer() called on a sub-builder registers in root graph."""
+        import numpy as np
+
+        root_graph = ir.Graph(
+            name="main",
+            inputs=[],
+            outputs=[],
+            nodes=[],
+            opset_imports={"": _default_opset_version},
+        )
+        root_builder = builder.GraphBuilder(root_graph)
+
+        def body(op, x):
+            tensor = ir.Tensor(np.array([1.0, 2.0], dtype=np.float32))
+            bias = op.builder.initializer(tensor, name="my_bias")
+            return op.Add(x, bias)
+
+        sub = root_builder.subgraph(body, inputs=[FLOAT[3]], outputs=[FLOAT[3]])
+        # Initializer should be in root, not subgraph
+        self.assertIn("my_bias", root_graph.initializers)
+        self.assertEqual(len(sub.initializers), 0)
+
     def test_lift_initializers_to_constants_converts_all(self):
         """lift_initializers_to_constants replaces initializers with Constant nodes."""
         graph = ir.Graph(