add tile ratio warning when there is class imbalance

@drewbo, should we add class imbalance warning?

When I create a bounding box for an image classification task, a building classifier. I set the `background_ratio` to 1 and assumed Label Maker will create a balance classes ratio. But in this case, the bounding box only contained building tiles, and I ended up only have 9 tiles are the background tiles out of 340 tiles. If we can add the class imbalance warning that will help to evaluate the training dataset.