Backend: Implement Accuracy Calculation using Confusion Matrix

[AngelinaZhai]

Implement the confusion matrix to calculate for the accuracy of the model. Create a separate function named get_accuracy() like the one we already have for this task.

Here are some resources to help you get started. They both have example codes:

• Some theoretical background and explanations of variations on Confusion Matrix
• Reading on Multi-label classification evaluation metrics in general

It would be ideal to have at least two members of the backend team working on this