[JIT] Add registry for backend lowering functions (#39552)
Summary:
**Summary**
This commit adds a registry for storing lowering functions for backends.
Instead of backends registering these lowering functions in separate C
extension modules, these will be registered in the Torch extension.
Backends are registered statically, so a registry is needed to hold
these lowering functions until Python bindings are created.
**Test Plan**
`python test/test_jit.py TestBackends`
```
Couldn't download test skip set, leaving all tests enabled...
..
----------------------------------------------------------------------
Ran 2 tests in 0.104s
OK
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39552
Reviewed By: mortzur
Differential Revision: D22033855
Pulled By: SplitInfinity
fbshipit-source-id: 05abf152274e5e51c37b6004886ea25bd4d33b80
