Refactor to_edge into smaller functions that can be reused (#3482)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/3482

The core logic of generating an edge program is reusable and will specifically be reused later on by the implementation of to_edge_transform_and_lower.

Reviewed By: mergennachin, mcr229

Differential Revision: D56401457

fbshipit-source-id: e994745f37408f02f2fc17236ef857eb06d57493