Add CI pipeline, unit tests, bug fixes, and contributing docs

[shreyaskamathkm]

Bug fixes:

• Fix misleading validator error message in cfg_dataclasses.py
• Replace print() with logger in model_helpers.py
• Fix hard-coded Path.cwd() in resnetv1b.py (now package-relative, overridable via FTNET_PRETRAINED_DIR)

Tooling:

• Add mypy, pytest, pytest-cov to dev dependencies in pyproject.toml
• Add [tool.mypy] and [tool.pytest.ini_options] config sections
• Add mypy pre-commit hook

CI/CD:

• Add .github/workflows/ci.yml with lint, type-check, and test jobs (Python 3.10 & 3.11)

Tests:

• Add tests/test_cfg_dataclasses.py
• Add tests/test_loss.py
• Add tests/test_model_helpers.py
• Add tests/test_schedulers.py
• Add tests/test_data_attributes.py

Docs:

• Add CONTRIBUTING.md with branch strategy, setup, and PR guidelines
• Add GitHub issue templates (bug report, feature request)
• Add GitHub PR template with checklist targeting develop