Fix the key used for extraction of delegate debug handle map in emitter (#493)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/493

In D49173918 while looking for the delegate map in the lowered_module metadata i ended up using the wrong key. It should be `debug_handle_map` and not `delegate_map`.

Reviewed By: Jack-Khuu

Differential Revision: D49620440

fbshipit-source-id: 473a6eb98dec712a45bf4b916397d6ccba2868eb