[Python][C++] Simplify/Validate PyObject refcount in udf.cc

[icexelloss]

Describe the bug, including details regarding any error messages, version, and platform.

In udf.cc, there is interaction between C++ and Python where the C++ code holds a user defined Python object inside a Kernel and invoke it over input data.

Due to this interaction, there are a number of Py_INCREF call as well as using OwnRef to manage the refcount for the PyObject (which represents the user defined function. The current code for managing it is either over complicated and/or bug where the C++ code might be over increasing the refcount. While this is not a huge issue (it probably cause the Python function to never be garbage collected), it could use some clean up / documentation as least to improve maintainability.

Appendix:

• Related thread: GH-35515: [C++][Python] Add non decomposable aggregation UDF #35514 (comment)

Component(s)

C++