Migrate examples/apple/... away from the torch:: namespace (#5405)

Summary:
Migrate examples/apple/... to use the new `executorch::` namespace.

Pull Request resolved: https://github.com/pytorch/executorch/pull/5405

Test Plan:
Build the examples using the instructions at
- https://github.com/pytorch/executorch/blob/main/examples/apple/coreml/README.md
- https://github.com/pytorch/executorch/blob/main/examples/apple/mps/README.md

Reviewed By: malfet, shoumikhin

Differential Revision: D62793645

Pulled By: dbort

fbshipit-source-id: b74b95b8ae87eeae40e57b195351c841c4a90885