Refactor DPP script (#168)

Refactors the DPP script primarily to improve code structure and readability.

- Modularizes the script with a `main()` and descriptive functions.
- Uses standalone functions for core logic e.g. `load_model`, `get_sampler`, and `get_valid_prompts`
- Separates AIU and CPU validation generation into distinct paths as well as metric-validation and token-reporting test types
- Adds high-level docstrings 

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Signed-off-by: Rafael Vasquez <rafvasq21@gmail.com>

Author: rafvasq