Use pytest-datadir in CI tests

[albertvillanova]

Use pytest-datadir in CI tests.

This PR improves the test infrastructure by adding the pytest-datadir dependency and updates two test cases to use the shared_datadir fixture for accessing test data. This change makes the tests more robust and portable by ensuring test data paths are handled consistently.

Test infrastructure improvements:

• Added pytest-datadir to both the test and dev dependency groups in pyproject.toml to provide a fixture for managing test data directories.

Test case updates:

• Updated test_train_with_set_chat_template_from_path in both tests/test_reward_trainer.py and tests/test_sft_trainer.py to use the shared_datadir fixture instead of a hardcoded path for loading template.jinja.

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.