Potential fix for code scanning alert no. 22179: Unused local variable

Co-authored-by: Copilot Autofix powered by AI <62310815+github-advanced-security[bot]@users.noreply.github.com>

Author: gramalingam

onnxscript/nn/_module_test.py

@@ -126,7 +126,7 @@ def inner_fn(op, x):
 
         def outer_fn(op, x):
             # Build a nested subgraph
-            _inner_graph = build_graph(
+            build_graph(
                 inner_fn,
                 inputs=[FLOAT[3]],
                 outputs=[FLOAT[3]],