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

[sweep-nightly]

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.

---

🎉 Latest improvements to Sweep:

• Sweep now uses the rope library to refactor Python! Check out Large Language Models are Bad at Refactoring Code. To have Sweep refactor your code, try sweep: Refactor <your_file>.py!
• Sweep finds and fixes bugs in your code by writing unit tests! Check out Having GPT-4 Iterate on Unit Tests like a Human.

---

💡 To get Sweep to edit this pull request, you can:

• Comment below, and Sweep can edit the entire PR
• Comment on a file, Sweep will only modify the commented file
• Edit the original issue to get Sweep to recreate the PR from scratch

[sweep-nightly]

Sandbox Executions

[sweep-nightly]

Apply Sweep Rules to your PR?

• Apply: All docstrings and comments should be up to date.
• Apply: Code should be properly formatted and indented.
• Apply: Variable and function names should be descriptive and follow a consistent naming convention.
• Apply: Imports should be organized and grouped together.
• Apply: There should be no unused imports or variables.
• Apply: Code should be properly commented and include docstrings for functions and classes.