Add unit tests for SamplingParams and CompilationConfig

[ChuanLi1101]

Summary

This PR adds a basic unit test framework to the ATOM project.

Changes

• Added tests/ directory with pytest-based unit tests
• Added tests for SamplingParams dataclass
• Added tests for CompilationConfig and CUDAGraphMode
• Updated pyproject.toml with pytest dev dependencies and configuration

Test Coverage

• test_sampling_params.py: Tests default values, custom values, and equality
• test_config.py: Tests CUDAGraphMode enum methods and CompilationConfig validation

How to Run

pip install pytest
pytest tests/ -v## Notes

• Tests are pure Python and do not require GPU hardware
• Compatible with both NVIDIA CUDA and AMD ROCm environments