[torchgen] Refactor types (#90589)
A retry of #89487. Accidentally closed.
## Split `torchgen.api.types` into `types_base`, `types` and `signatures`.
In `types_base`:
* Created base class `CType`. `BaseCType` and `ConstRefCType` etc are inheriting `CType`.
* Only keep abstract type model definitions, such as `BaseCppType`.
In `types`:
* Define `BaseCppType` with `at` and `c10` namespaces.
* All the signatures using these types.
In `signatures`:
* Define all the signatures.
In `__init__`:
* `from ... import *`, suppress flake8 error.
Differential Revision: [D41455634](https://our.internmc.facebook.com/intern/diff/D41455634/)
**NOTE FOR REVIEWERS**: This PR has internal Meta-specific changes or comments, please review them on [Phabricator](https://our.internmc.facebook.com/intern/diff/D41455634/)!
Pull Request resolved: https://github.com/pytorch/pytorch/pull/90589
Approved by: https://github.com/iseeyuan
