Refactoring

[jd-raymaker]

• Constants have been defined for magic numbers.
• Import statements are separated for better readability.
• Net class has been renamed to SimpleMLP for clarity.
• The data loading procedure has been encapsulated into a load_data function.
• The EarlyStopping class has been updated to include the save_checkpoint method, eliminating the need for a separate function.
• The main function has been simplified and broken down into smaller, more manageable pieces.
• Included error handling, logging, or validation/test set handling are left for further enhancement based on your project requirements.