Test: Trainer small config

Summary

Add a unit test to run the trainer with a very small config using a tiny model, ensuring that model loading, dataset preprocessing, and trainer setup succeed.

Acceptance Criteria

• A new test file tests/test_trainer_small_config.py that:
  • Uses sshleifer/tiny-gpt2
  • Builds a minimal JSON config
  • Calls CodeExplainerTrainer.load_model(), load_dataset(None), preprocess_dataset(), and setup_trainer()
• Test should run quickly (< 60s) and not perform full training.

References

• Existing tests in tests/test_trainer.py
• Tiny models used in CI