Parsing reference outputs from etrecord and save in inspector and event block (#1346)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1346

The inspector instance would hold all reference outputs in a dict, keyed by method name. We assume there's only the "forward" key for now. Each event block instance would just hold the one output that's indexed by `event_block.bundled_input_index`.

Also did a small refactoring because I introduced a new type alias ProgramOutput.

Reviewed By: tarun292, Jack-Khuu

Differential Revision: D51290998

fbshipit-source-id: 02a4bfb6d0c965058a521f48d41f354acc3281b6