Refactor extract_segments -> extract_delegate_segments (#1337)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1337

In D51596390 we introduce `extract_constant_segment`. This refactor updates `extract_segments` to specifically `extract_delegate_segments` now that we have different segments being extracted.

Reviewed By: larryliu0820

Differential Revision: D51725073

fbshipit-source-id: a843b01d102b05d0ebc635bfac45f29e93731976