Fix for the IOU metric (#12709)

* Fixed mean iou case when a class does not appear in the labels nor in the prediction.

* Added 3 tests for both mean_iou and streaming_mean_iou, 2 of which would fail with the previous code and one to make sure the behavior is still correct in the normal case. Fixed broken tests as well.

* Added check for div by 0 in iou metric.

* Add space around operator

As per style guide.