Move constants to constant_segment (#1343)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1343

* Add `extract_constant_segment` field, defaults to `False`, add `_extract_constant_segment(..)` to move constants to segment
* Segments are extracted twice (delegates, constants), so we create a copy of the program inside `serialize_pte_binary` to use for modification.

Potential future refactor: rename extract_segments -> extract_delegate_segments

Reviewed By: dbort

Differential Revision: D51596390

fbshipit-source-id: 63a9e4843379851097d262e3fc1b21f8e88d17d5