[torchgen] Move Executorch unboxing logic into torchgen (#90098)
This PR adds `unboxing.py` which converts a `EValue` (similar to `IValue`) to its corresponding C++ type, based on the `ExecutorchCppSignature`.
Added unit tests to it in `test_executorch_unboxing.py`. Notice that this unboxing logic should work for both ATen types and Executorch types, hence the unit tests are parametrized.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/90098
Approved by: https://github.com/ezyang
