Fail earlier if chains are not present (#1545)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1545

D52451740 made `n_chains_` default to 0 when the chains field is missing, but I just noticed that later on we fail when `n_chains_` is zero. So, just fail immediately if the field is missing.

Reviewed By: JacobSzwejbka

Differential Revision: D52554052

fbshipit-source-id: 40f6e5a0839c418736ef24a82e726ed2cc8a15f0