Updates name template for compatibility with Cython

[george-kuanli-peng]

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The current generated code cannot be compiled by Cython with errors like this:

[1/3] Cythonizing XXX.py

Error compiling Cython file:
------------------------------------------------------------
...
        if False:
            ci = NotImplemented

        def _walrus_wrapper_ci_54f2f2c3aa5e4a6488fc2b75e012a2b0(expr):
            """Wrapper function for assignment expression."""
            nonlocal ci
            ^
------------------------------------------------------------

XXX.py:125:12: no binding for nonlocal 'ci' found

This PR generates the following code, which introduces some runtime cost but works with Cython:

        if 'ci' not in dir():
            ci = NotImplemented

        def _walrus_wrapper_ci_54f2f2c3aa5e4a6488fc2b75e012a2b0(expr):
            """Wrapper function for assignment expression."""
            nonlocal ci