Remove `torch::` references from devtools/example_runner (#5495)

Summary:
Use the names in the new `executorch::` namespace.

Pull Request resolved: https://github.com/pytorch/executorch/pull/5495

Test Plan:
```
./examples/devtools/build_example_runner.sh
```

Reviewed By: larryliu0820

Differential Revision: D63047148

Pulled By: dbort

fbshipit-source-id: e0e3af1c130aaf409ecc142c28d75f0a44d88fa3