Update executorch serialization (#1397)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1397

Executorch exir's serialization has diverged a bit from export's due to some changes from the export side.

Reviewed By: ydwu4

Differential Revision: D52054112

fbshipit-source-id: 72e5c06226edc733c4944c0e738750a253a6af4d