Refactor: Add missing documentation to blob/develop/label_studio/ml/models.py

**Issue: Lack of Documentation in models.py**

This fix will align the code more with the Coding Standards in the CONTRIBUTING.md file, per this rule: "Use comments to describe code blocks."

When looking through the models.py file (https://github.com/HumanSignal/label-studio/blob/develop/label_studio/ml/models.py), I noticed some missing documentation. The MLBackend class has empty documentation left as """ """.

I also notice the save function has a detailed description, whereas other intricate functions such as update_state, _get_predictions_from_ml_backend_one_by_one, _get_predictions_from_ml_backend, predict_tasks, and the majority of functions on the page to not have descriptions.

This will making it easier for new contributors to understand what the parts of the code does, without needing to read them line by line.