Add CNN class for MNIST handling and import it to main.py

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Description

This PR adds a new CNN class in a new file named cnn.py to handle the MNIST dataset. The CNN class is implemented using PyTorch and includes the model definition and training code. The trained model is then imported into main.py for further use.

Summary of Changes

• Created a new file src/cnn.py to contain the CNN class.
• Imported necessary PyTorch modules in cnn.py.
• Defined a new class CNN in cnn.py that inherits from torch.nn.Module.
• Implemented the __init__ method in CNN class to define the layers of the CNN.
• Implemented the forward method in CNN class to perform the forward pass of the CNN.
• Defined a function train_model in cnn.py to handle the training loop.
• Modified src/main.py to import the CNN class and train_model function from cnn.py.
• Instantiated the CNN class in main.py to create the model.
• Defined a loss function and an optimizer in main.py.
• Called the train_model function in main.py to train the model.

Please review and merge this PR to incorporate the changes.

Fixes #9.

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