docs(export): fix runnable Exporter examples

[onion-hong]

The Exporter class examples pass a filename string to an API that expects a model module. Running the doctest on upstream b44ea12 fails with AttributeError: 'str' object has no attribute 'named_modules' during export preflight.

Build a randomly initialized model from YAML and pass its .model to both examples. Small TorchScript exports keep the examples runnable without downloading pretrained weights or requiring an additional export backend. Capture the returned path so the doctest does not depend on its printed representation. This changes documentation only.

Validation:

• Reproduced the failure on upstream b44ea12.
• python -m pytest --doctest-modules ultralytics/engine/exporter.py -q: 1 passed after the change, including real TorchScript exports.
• ruff format --check ultralytics/engine/exporter.py and git diff --check passed.
• Ruff lint still reports pre-existing diagnostics outside the changed examples; codespell is not installed in the validation environment.