[BE] Cleanup python_function.cpp (#66296)
Summary:
- Delete unused `var_input_idx`
- Fix `uninitialized variable` clang-tidy warning by setting `PyObject* input` to PyNone
Pull Request resolved: https://github.com/pytorch/pytorch/pull/66296
Reviewed By: janeyx99
Differential Revision: D31491016
Pulled By: malfet
fbshipit-source-id: 08267144be0cd049d122580cdf81cf586c3e30a6
