wrap dict in struct (#1555)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1555

Seeing error: `Error: 'dict' value has no field or method 'to_json'`

This diff patches the error by wrapping the dict into a struct.

Reviewed By: larryliu0820

Differential Revision: D52610719

fbshipit-source-id: 459f77d0aa04a6aff2f98478e52b97877463a42d