[jit] Fix ResolutionCallback definition (#51089)
Summary:
`ResolutionCallback` returns `py::object` (i.e. `Any`) rather than `py::function` (i.e. `Callable`)
Discovered while debugging test failures after updating pybind11
This also makes resolution code slightly faster, as it eliminates casts from object to function and back for every `py::object obj = rcb_(name);` statement.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51089
Reviewed By: jamesr66a
Differential Revision: D26069295
Pulled By: malfet
fbshipit-source-id: 6876caf9b4653c8dc8e568aefb6778895decea05
