[torchgen] Extract out schema registration logic into a function (#80780)
Summary:
A followup to #78015 and #79733. In those PRs I introduced custom namespace support into:
* `Register<DispatchKey>.cpp`
* `RegisterSchema.cpp`
* `NativeFunctions.h`
This PR extracts out logic that generates schema registration code (used in `RegisterSchema.cpp`) into a function so that it can be easily tested and reused. Added unit test to cover the logic as well.
Test Plan: Rely on newly added unit tests.
Differential Revision: D37581186
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80780
Approved by: https://github.com/iseeyuan
