Add weights to model outputs (#4302)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4302

Added a pass that flags weights with associated gradients and adds them to the models outputs. Updated the emitter to handle that something could be both a 'constant' and have an allocation_info

Reviewed By: tarun292

Differential Revision: D59929421

fbshipit-source-id: 6c3c85d260754b7c8271afc1f89877af68943fab