[torchgen] Add CI job to make sure torchgen works for Executorch op registration (#89596)
## Job
Test running on most CI jobs.
## Test binary
* `test_main.cpp`: entry for gtest
* `test_operator_registration.cpp`: test cases for gtest
## Helper sources
* `operator_registry.h/cpp`: simple operator registry for testing purpose.
* `Evalue.h`: a boxed data type that wraps ATen types, for testing purpose.
* `selected_operators.yaml`: operators Executorch care about so far, we should cover all of them.
## Templates
* `NativeFunctions.h`: for generating headers for native functions. (not compiled in the test, since we will be using `libtorch`)
* `RegisterCodegenUnboxedKernels.cpp`: for registering boxed operators.
* `Functions.h`: for declaring operator C++ APIs. Generated `Functions.h` merely wraps `ATen/Functions.h`.
## Build files
* `CMakeLists.txt`: generate code to register ops.
* `build.sh`: driver file, to be called by CI job.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/89596
Approved by: https://github.com/ezyang
