[torchgen] Introduce Executorch types and signatures (#90591)
Retry of #89595. Accidentally closed.
## Forked `BaseCppType`
Created a module for Executorch: `torchgen.executorch`.
In `torchgen.executorch.api.types.types`:
* Define `BaseCppType` with `torch::executor` namespace.
In `torchgen.executorch.api.et_cpp`:
* Help generate `NamedCType` for `ExecutorchCppSignature` arguments.
In `torchgen.executorch.api.types.signatures`:
* Define the signature using these types. (`ExecutorchCppSignature`)
In `torchgen.executorch.api.types.__init__`:
* Suppress flake8 error for `import *`.
Differential Revision: [D41501836](https://our.internmc.facebook.com/intern/diff/D41501836/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/90591
Approved by: https://github.com/iseeyuan
