[5694695][AutoCast] Preserve outer scope variable types in subgraphs (#717)

## What does this PR do?

**Type of change:** Bug fix

**Overview:** 
When clearing type information for shape inference, preserve value_info
for outer scope variables in subgraphs. Previously, all value_info
entries were cleared indiscriminately, causing shape inference failures
when subgraph nodes referenced outer scope variables.


## Testing
pytest
tests/unit/onnx/autocast/test_precisionconverter.py::test_if_subgraph_outer_scope_type_preservation

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- **Is this change backward compatible?**: Yes
- **Did you write any new necessary tests?**: Yes
- **Did you add or update any necessary documentation?**: N/A
- **Did you update
[Changelog](https://github.com/NVIDIA/Model-Optimizer/blob/main/CHANGELOG.rst)?**:
No

Signed-off-by: Gal Hubara Agam <96368689+galagam@users.noreply.github.com>

Author: galagam

modelopt/onnx/autocast/precisionconverter.py

@@ -283,6 +283,8 @@ def _clear_types_and_shapes_recursive(
         """Recursively clear type/shape information for a graph and all its subgraphs.
 
         This is necessary for control flow operators (Scan, If, Loop) which have subgraphs.
+        For subgraphs, preserve value_info for outer scope variables (not produced by nodes in subgraph).
+        For main graph, clear all value_info.
 
         Args:
             graph: The ONNX graph to clear types and shapes for.
@@ -303,13 +305,27 @@ def _clear_callback(g: onnx.GraphProto, parent: onnx.NodeProto, is_sub: bool) ->
                             if d.dim_value:
                                 inp.type.tensor_type.shape.dim[idx].dim_param = "unk"
 
-            # Clear type/shape information for intermediates and outputs
-            for vi in g.value_info:
-                vi.type.tensor_type.elem_type = onnx.TensorProto.UNDEFINED
-                for idx, d in enumerate(vi.type.tensor_type.shape.dim):
-                    if d.dim_value:
-                        vi.type.tensor_type.shape.dim[idx].dim_param = "unk"
+            if is_sub:
+                # Identify which tensors are produced by nodes in this subgraph
+                subgraph_outputs = set()
+                for node in g.node:
+                    subgraph_outputs.update(node.output)
+
+                # Clear value_info only for intermediates produced by nodes in this subgraph
+                for vi in g.value_info:
+                    if vi.name in subgraph_outputs:
+                        vi.type.tensor_type.elem_type = onnx.TensorProto.UNDEFINED
+                        for idx, d in enumerate(vi.type.tensor_type.shape.dim):
+                            if d.dim_value:
+                                vi.type.tensor_type.shape.dim[idx].dim_param = "unk"
+            else:
+                for vi in g.value_info:
+                    vi.type.tensor_type.elem_type = onnx.TensorProto.UNDEFINED
+                    for idx, d in enumerate(vi.type.tensor_type.shape.dim):
+                        if d.dim_value:
+                            vi.type.tensor_type.shape.dim[idx].dim_param = "unk"
 
+            # Clear outputs for both main graph and subgraphs
             for out in g.output:
                 out.type.tensor_type.elem_type = onnx.TensorProto.UNDEFINED
                 for idx, d in enumerate(out.type.tensor_type.shape.dim):

tests/unit/onnx/autocast/test_precisionconverter.py

@@ -1520,3 +1520,79 @@ def test_if_subgraph_mixed_precision_boundary(model_with_if_subgraph, low_precis
     # Verify a cast was inserted between If output and Add input
     cast_nodes = [n for n in converted_model.graph.node if n.op_type == "Cast"]
     assert len(cast_nodes) > 0, "Should have cast nodes for mixed precision"
+
+
+@pytest.fixture
+def model_with_if_outer_scope_reference():
+    """Create a minimal model where If subgraphs reference outer scope variables.
+
+    This tests that subgraph value_info for outer scope variables is preserved during type clearing.
+    """
+    # Main graph inputs/outputs
+    x = helper.make_tensor_value_info("X", TensorProto.FLOAT, [2, 4])
+    condition = helper.make_tensor_value_info("condition", TensorProto.BOOL, [])
+    y = helper.make_tensor_value_info("Y", TensorProto.FLOAT, [2, 4])
+
+    # Create "then" branch: Identity on X from outer scope
+    then_y = helper.make_tensor_value_info("then_y", TensorProto.FLOAT, [2, 4])
+    then_identity = helper.make_node("Identity", ["X"], ["then_y"], name="then_identity")
+    then_graph = helper.make_graph([then_identity], "then_branch", [], [then_y])
+    # Add X to value_info - this is what needs to be preserved
+    then_graph.value_info.extend([helper.make_tensor_value_info("X", TensorProto.FLOAT, [2, 4])])
+
+    # Create "else" branch: Identity on X from outer scope
+    else_y = helper.make_tensor_value_info("else_y", TensorProto.FLOAT, [2, 4])
+    else_identity = helper.make_node("Identity", ["X"], ["else_y"], name="else_identity")
+    else_graph = helper.make_graph([else_identity], "else_branch", [], [else_y])
+    else_graph.value_info.extend([helper.make_tensor_value_info("X", TensorProto.FLOAT, [2, 4])])
+
+    # Create If node and main graph
+    if_node = helper.make_node(
+        "If", ["condition"], ["Y"], name="if_node", then_branch=then_graph, else_branch=else_graph
+    )
+    main_graph = helper.make_graph([if_node], "model_with_outer_scope", [x, condition], [y])
+
+    model = helper.make_model(main_graph, producer_name="model_with_outer_scope")
+    model.opset_import[0].version = 20
+    onnx.checker.check_model(model)
+
+    model = onnx_utils.infer_shapes(model)
+    value_info_map, initializer_map, node_to_init_map = utils.setup_mappings(model)
+    return model, value_info_map, initializer_map, node_to_init_map
+
+
+@pytest.mark.parametrize("low_precision_type", ["fp16", "bf16"])
+def test_if_subgraph_outer_scope_type_preservation(
+    model_with_if_outer_scope_reference, low_precision_type
+):
+    """Test that outer scope variable types are preserved in If subgraphs during conversion.
+
+    Without preserving X's value_info in subgraphs, shape inference fails with
+    "Element type of input 0 unknown".
+    """
+    model, value_info_map, initializer_map, node_to_init_map = model_with_if_outer_scope_reference
+
+    converter = PrecisionConverter(
+        model,
+        value_info_map,
+        initializer_map,
+        node_to_init_map,
+        keep_io_types=True,
+        low_precision_type=low_precision_type,
+    )
+
+    converted_model = converter.convert(high_precision_nodes=["if_node"], low_precision_nodes=[])
+    onnx.checker.check_model(converted_model)
+
+    # Verify X's value_info is preserved in both subgraphs
+    if_node = next(n for n in converted_model.graph.node if n.op_type == "If")
+    then_branch = next(attr.g for attr in if_node.attribute if attr.name == "then_branch")
+    else_branch = next(attr.g for attr in if_node.attribute if attr.name == "else_branch")
+
+    then_x_info = [vi for vi in then_branch.value_info if vi.name == "X"]
+    else_x_info = [vi for vi in else_branch.value_info if vi.name == "X"]
+
+    assert len(then_x_info) > 0, "X value_info should be preserved in then branch"
+    assert len(else_x_info) > 0, "X value_info should be preserved in else branch"
+    assert then_x_info[0].type.tensor_type.elem_type != onnx.TensorProto.UNDEFINED
+    assert else_x_info[0].type.tensor_type.elem_type != onnx.TensorProto.UNDEFINED