Refactor training loop from script to class

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Description

This PR refactors the training loop from a script to a class in order to improve code modularity and maintainability. The training loop is currently implemented in the src/main.py file as a script, which makes it difficult to reuse and test. By refactoring it into a class, the code becomes more modular and easier to manage.

Summary of Changes

• Created a new class named MNISTTrainer in the src/main.py file.
• Moved the data loading and preprocessing code into a method named load_data in the MNISTTrainer class.
• Moved the model definition code into a method named define_model in the MNISTTrainer class.
• Created a method named train in the MNISTTrainer class to implement the training loop.
• Created a method named evaluate in the MNISTTrainer class to evaluate the model on the test data.
• Created a method named save_model in the MNISTTrainer class to save the trained model to a file.
• Created a method named load_model in the MNISTTrainer class to load a trained model from a file.
• Updated the Net class definition to be inside the MNISTTrainer class.
• Updated the script at the end of the src/main.py file to use the MNISTTrainer class.
• Updated the import statement in the src/api.py file to import the MNISTTrainer class.
• Updated the code in src/api.py to use the load_model method of the MNISTTrainer class.
• Updated the README.md file to reflect the changes in the code and provide instructions on how to use the MNISTTrainer class.

Please review and merge this PR. Thank you!

Fixes #6.

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Rollback Files For Sweep

• Rollback changes to src/main.py
• Rollback changes to src/api.py
• Rollback changes to README.md

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