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 modularity, readability, and testability. The training loop, which was previously located in the main.py file, has been encapsulated within a new class named MNISTTrainer. This class has methods for loading the MNIST data, preprocessing it, and training the model. The api.py file has also been updated to import the MNISTTrainer class instead of the Net class.

Summary of Changes

• Created a new class named MNISTTrainer in main.py
• Moved the code for loading the MNIST 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 for training the model
• Created a new method named run to run the entire training process
• Updated the script in main.py to create an instance of MNISTTrainer and call the run method
• Updated the import statement in api.py to import the MNISTTrainer class
• Updated the code in api.py to create an instance of MNISTTrainer and call the run method

Fixes #6.

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Rollback Files For Sweep

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

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