Check for missing method-related field before creating MethodMeta (#1507)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1507

Some MethodMeta methods don't return Error, assuming that the underlying data is valid. Ensure that the underlying ExecutionPlan is valid for those fields before returning a MethodMeta.

For fields whose accessors do return Error or Result, we can check them at the time they're called and return non-fatally then.

Reviewed By: lucylq

Differential Revision: D52451736

fbshipit-source-id: 24e7f603cd53274b46cc6b8ff89f8b084e218c8a