Unlabeled json creator

[PeterVennerstrom]

Thanks for your great work!!!

Added an unlabeled json creator to tools/datasets. This creates the minimum necessary coco json to train from a folder of unlabeled images.

Added an assertion to semi_sampler.py. When either the train or unlabeled set contains only images of one aspect ratio group h > w or w > h it would otherwise throw the semi_sampler.py line 187: assert len(indices) == len(self) assertion error.

From reading the issues it appears there are multiple causes and solutions to the semi_sampler.py line 187: assert len(indices) == len(self) assertion error. This new assertion isolates errors related to aspect ratio groups.

[ghost]

All CLA requirements met.