Return a CType instead of a string for returns, beef up CType (#55046)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55046
Updating `returns` in the codegen to return a CType instead of a raw string.
This has benefit of putting all stringifying logic through CType, which is useful in the followup PR when I add namespaces.
I also added new CTypes for other templated C++ types: array, vector and tuple. Mostly because it makes the namespacing logic in the next PR significantly easier. It also seems more natural to me that `BaseCType` shouldn't represent specializations of templated types.
There's a little bit of weirdness, types that are currently *only* used for returns, i.e. `TupleCType`. Returns aren't named, so I opted not to give it one- so we can add it in later if we discover that we need it.
Test Plan: Imported from OSS
Reviewed By: bhosmer
Differential Revision: D27708348
Pulled By: bdhirsh
fbshipit-source-id: 230b210c3e53be1bd362105fbea8451055dc59a8
