Enable Free-threaded Python (PEP 703) support for Python 3.13t+

[tianleiwu]

This PR resolves the RuntimeWarning encountered when importing onnxruntime in free-threaded Python environments (e.g., Python 3.13t, 3.14t).

Previously, the module did not explicitly declare that it could run safely without the GIL, causing the interpreter to re-enable the GIL at runtime.

The Warning

RuntimeWarning: The global interpreter lock (GIL) has been enabled to load module
'onnxruntime.capi.onnxruntime_pybind11_state', which has not declared that it can
run safely without the GIL.

Changes

1. Build System (cmake/onnxruntime_python.cmake)

• Added robust detection logic to check if the current Python interpreter is free-threaded.

Detection strategy:

• Primary: Check sysconfig.get_config_var('Py_GIL_DISABLED') (PEP 703 standard).

• Fallback: Inspect ABI flags (ABIFLAGS or SOABI) for the t suffix (e.g., cp313t), handling cases on Windows where config variables may return None or empty strings.

• Caching: Cache the result as ORT_PYTHON_FREE_THREADED_DETECTED (BOOL) to avoid re-running detection on every configure.

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2. C++ Source (onnxruntime/python/onnxruntime_pybind_module.cc)

• Updated the PYBIND11_MODULE definition to conditionally enable the GIL-free slot.
• Uses py::mod_gil_not_used() (available in pybind11 ≥ 2.13) when Py_GIL_DISABLED is defined.

---

Verification

Environment:

• Python 3.14t (Free-threaded)
• Windows & Linux

Before Change

python3.14t -c "import onnxruntime"

Resulted in the RuntimeWarning shown above.

After Change

1. Build log confirms correct detection:

-- Python_EXECUTABLE: D:\py314t\Scripts\python.exe
-- Py_GIL_DISABLED=1 detected: Enabling free-threaded support for onnxruntime_pybind11_state

2. Runtime behavior:

python3.14t -c "import onnxruntime"

Runs silently with no warnings, confirming the module loads successfully with the GIL disabled.

Related Issue

• [Build] Suppress RuntimeWarning about GIL enabling when importing onnxruntime on Free-threaded Python (3.13t) #26780