Clean up lowering APIs (#3690)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/3690

Changes some types in `export_to_edge`, add an `export_to_cadence` function, and refactor some calls. Mostly preemptive work before adding a lot of AoT passes.

Reviewed By: dulinriley, zonglinpengmeta

Differential Revision: D57579738

fbshipit-source-id: 8c7975488636bd946894f7f7e62da7e76eb29c65