Refactor training loop from script to class

[sweep-nightly]

Description

This PR refactors the training loop from a script to a class in order to improve code organization and reusability. The existing code in src/main.py is encapsulated into a new class named MNISTTrainer, which includes methods for loading and preprocessing the data, defining the model, training the model, and saving the trained model.

Summary of Changes

• Created a new class named MNISTTrainer in src/main.py.
• Moved the code for loading and preprocessing the data into a method named load_data.
• Moved the code for defining the model into a method named define_model.
• Created a new method named train_model to implement the training loop.
• Created a new method named save_model to save the trained model.
• Updated the script to create an instance of MNISTTrainer and call the methods in the correct order.
• Added proper documentation with docstrings and comments for all methods.

Please review and merge this PR. Thank you!

Fixes #6.

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• Rollback changes to src/main.py
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