Emit lowered module (#89)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/89

It has been a pending task for a while, as a follow up on https://fb.workplace.com/groups/536346827621174/permalink/665126474743208/ we want the lowered backend module to be **runnerable**, **emittable**, and **retracable**. This diff makes the lowered backend module emittable without the need to composite with other modules

It will the easiest the flow for backend developer to try lower one op to a backend via delegate.

Reviewed By: angelayi

Differential Revision: D47803806

fbshipit-source-id: aae31442591e497976359413a088cb2460107627