Hide internal-only Method methods (#1593)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1593

No one should be messing with Method values or value indices directly. Make these methods private instead of deprecated.

Remove the unused `values_size()` because the internal code compares against `n_value_` directly.

Reviewed By: tarun292

Differential Revision: D52717552

fbshipit-source-id: cede311614ddde77a2bcaf28f22da391ca3843e1