add begin/end to ExecuTorch pytree::arr (#8300)

Pull Request resolved: https://github.com/pytorch/executorch/pull/7653

Allows use of range-for.
ghstack-source-id: 265152271
@exported-using-ghexport

Differential Revision: [D68166302](https://our.internmc.facebook.com/intern/diff/D68166302/)

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