Add fake mode in verifier (#5805)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/5805

Hopefully fixes https://fb.workplace.com/groups/pytorch.edge.users/permalink/1605630670307220/

Reviewed By: larryliu0820

Differential Revision: D63734251

fbshipit-source-id: 854750227b64125a3609245c6cfcbff26b71f26a