Refactor training loop from script to class

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Description

This PR refactors the training loop from a script to a class for better organization and reusability. The existing code in src/main.py is encapsulated into a new class named MNISTTrainer. The class is responsible for loading and preprocessing the MNIST data, defining the PyTorch model, training the model, and saving/loading the model for future use.

Summary of Changes

• Created a new class named MNISTTrainer in src/main.py.
• Moved the code for loading and preprocessing the MNIST data into the load_data method of the MNISTTrainer class.
• Moved the code for defining the PyTorch model into the define_model method of the MNISTTrainer class.
• Implemented the train method in the MNISTTrainer class, which contains the training loop for the model.
• Implemented the save_model method in the MNISTTrainer class, which saves the trained model to a file.
• Implemented the load_model method in the MNISTTrainer class, which loads the model from a file.
• Updated the code in src/api.py to use the load_model method of the MNISTTrainer class for loading the model.

Please review and merge this PR to complete the requested refactoring of the training loop.

Fixes #6.

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Rollback Files For Sweep

• [ ] Rollback changes to src/api.py
• [ ] Rollback changes to src/main.py
• [ ] Rollback changes to src/api.py
• [ ] Rollback changes to src/main.py

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