Move examples/models/... out of the `torch` namespace (#5318)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/5318

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.

I made one non-mechanical change in llama_tiktoken.h: we should always use `enum class` to avoid polluting the parent namespace, and enum values should follow UpperSnakeCase.

Tests:
Should be a no-op; CI passes.

Prints no output:
```
find examples/models -type f | xargs grep "namespace torch"
find examples/models -type f | xargs grep "namespace executorch"
```

Build llava_main:
```
bash .ci/scripts/test_llava.sh
```

Reviewed By: larryliu0820

Differential Revision: D62591348

fbshipit-source-id: 8da987245bf777ced16000f681b54652939cef47