Add CNN class for MNIST and integrate it into main.py and api.py

[sweep-nightly[bot]]

Description

This pull request adds the CNN class for MNIST and integrates it into the main.py and api.py files. It also updates the import statements and removes unnecessary imports.

Summary

• Added import statement for the CNN class from cnn.py in api.py
• Updated import statements in api.py and main.py
• Removed unnecessary imports in api.py and main.py
• Replaced the Net class with the CNN class in main.py
• Updated the model initialization in main.py
• Replaced the model initialization in api.py with the CNN class
• Updated the optimizer initialization in main.py
• Updated the criterion initialization in main.py

Fixes #9.

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