[Models] Notebook with metrics in the WorldFloods test dataset

[gonzmg88]

Things to take into account:

• [x] Function that given a pytorch dataset returns a confusion matrix (len(dataset), num_class, num_class).
• [x] Function that takes that and output an JSON of average metrics
• [ ] Consider that eventually we might want to stratify results by permanent/non-permanent water
• [ ] Consider that eventually we might want to save the output preds for viz.
• [ ] Consider functionality to measure performance on a per-flood event basis