[FuncTorch] Fix compilation with -Werror (#83463)
- Fixed signed-unsigned compares
- Get rid of unused variables
- Typecast to `PyCFunction` via `(void*)`
- `ssize_t` is not a valid type on Win32
Pull Request resolved: https://github.com/pytorch/pytorch/pull/83463
Approved by: https://github.com/zou3519
