add util to get non lower nodes (#1088)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1088

Add a util function to get the non lowered nodes from the graph so it's easy to do post processing

Reviewed By: tarun292

Differential Revision: D50507998

fbshipit-source-id: fa8770218185df7db3f35454b7538864df40b182