Fix mutable class default lint warning in mha_scale_test

Use tuples instead of lists for class-level _3D and _OUT constants.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: gramalingam

onnxscript/rewriter/ort_fusions/mha_scale_test.py

@@ -83,8 +83,8 @@ def _get_mha_node(self, model: ir.Model) -> ir.Node | None:
                 return node
         return None
 
-    _3D = [FLOAT["B", "S", _D]] * 3
-    _OUT = [FLOAT["B", "S", _D]]
+    _3D = (FLOAT["B", "S", _D],) * 3
+    _OUT = (FLOAT["B", "S", _D],)
 
     def _check_numerical_equivalence(self, model: ir.Model, inputs: dict, expected_count: int):
         original_output = test_utils.ort_run("Original", model, inputs)