Move examples/mediatek out from under the torch namespace (#5478)

Summary:
The code under examples/... is a proxy for user code, and users should never declare code under the `torch::` or `executorch::` namespaces.

Move this code under the `example::` namespace to make it more clear that users should use their own namespaces when writing code like this.

Pull Request resolved: https://github.com/pytorch/executorch/pull/5478

Test Plan: - Built using the instructions at https://github.com/pytorch/executorch/blob/main/examples/mediatek/README.md

Reviewed By: JacobSzwejbka, cccclai

Differential Revision: D62992974

Pulled By: dbort

fbshipit-source-id: b01f1b33d2853a0555ae19d79769a5bb6d0ba853