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

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Description

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

Summary

• Added CNN class in cnn.py
• Updated import statements in api.py and main.py
• Replaced Net class with CNN class in main.py
• Removed unnecessary import statements from api.py

Fixes #9.

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