Switch CI type checking to ty and fix typing issues

[RajeevAtla]

Summary

• replace the CI mypy step with ty and install the new checker alongside existing tooling
• harden optional torch imports across models, training helpers, demos, and scripts with TYPE_CHECKING shims and safe fallbacks that satisfy ty
• tighten dataset utilities, evaluation metrics, and tests with clearer type annotations and runtime conversions uncovered by the new checker

Testing

• ty check --python $(python -c 'import sys; print(sys.executable)')
• ruff check .

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https://chatgpt.com/codex/tasks/task_e_68f5cac3a4048328b8424391f9555a7d