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 organization and reusability. The existing code in src/main.py is refactored into a new class named MNISTTrainer. The Net class is also updated to include a train method that utilizes the MNISTTrainer class.

Summary of Changes

• Created a new class MNISTTrainer in src/main.py to handle the loading, preprocessing, and training of the MNIST data.
• Moved the code for loading and preprocessing the data into the __init__ method of MNISTTrainer.
• Added a train method to MNISTTrainer that contains the training loop for the model.
• Updated the Net class in src/main.py to include a train method that creates an instance of MNISTTrainer and calls its train method.
• Updated the predict function in src/api.py to use the new train method of the Net 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
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