Temporarily ignore flaky mha test (pytorch#6790)

Author: jackzhxng

pytest.ini

@@ -39,6 +39,7 @@ addopts =
     backends/xnnpack/test
     # extension/
     extension/llm/modules/test
+    --ignore=extension/llm/modules/test/test_mha.py
     extension/pybindings/test
     # Runtime
     runtime