Refactor training loop from script to class

[sweep-nightly]

Description

This PR refactors the training loop from a script to a class for better organization and reusability. The training loop, which is currently in the main.py file, is encapsulated in a new class named MNISTTrainer. The code for loading and preprocessing the MNIST dataset is moved into a method named load_data, and the code for defining the PyTorch model is moved into a method named define_model. Additionally, a new method named train is added to MNISTTrainer to contain the training loop.

Summary of Changes

• Created a new class named MNISTTrainer in main.py.
• Moved the code for loading and preprocessing the MNIST dataset into a method named load_data in MNISTTrainer.
• Moved the code for defining the PyTorch model into a method named define_model in MNISTTrainer.
• Added a new method named train in MNISTTrainer to contain the training loop.
• Updated the import statement in api.py to import MNISTTrainer instead of Net.
• Updated the code for loading the model in api.py to use the load_data and define_model methods of MNISTTrainer.

Fixes #6.

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Rollback Files For Sweep

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

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