Replace pytree_assert with production pytree_check. Remove pytree_unreachable

Pull Request resolved: https://github.com/pytorch/executorch/pull/7655

When handling untrusted input, it's not appropriate to use debug-only checks; we should be checking in prod as these are not programmer errors. pytree_unreachable was similarly being used for input validation.
ghstack-source-id: 265152270

Differential Revision: [D68166301](https://our.internmc.facebook.com/intern/diff/D68166301/)

Co-authored-by: Github Executorch <github_executorch@arm.com>