[torchgen] Introduce Executorch types and signatures (#90781)
Retry of #90591, which is a retry of #89595. Reverted due to dependency PR breaking internal fbcode.
## 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 *`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/90781
Approved by: https://github.com/ezyang
