Refactor checkpoint conversion script for improved readability and efficiency

[tdas3001]

• Replaced os.path.join with Pathlib for improved readability and efficiency
• Used Path.glob() instead of glob(os.path.join(...)) for file operations
• Simplified directory creation with Path.mkdir(parents=True, exist_ok=True)
• Refactored parameter splitting logic to remove unnecessary loops and redundant conditions
• Used direct indexing for expert assignment instead of iterating over all model parallel indices
• Replaced .narrow() with Python slicing for improved readability
• Added an assertion to ensure mp > 0 before proceeding
• Reorganized key mapping logic for better readability and maintainability
• Optimized name modifications using a single chained .replace() operation
• Ensured that only necessary operations are performed when processing model parameters