Created by: baiyfbupt

Fix https://github.com/PaddlePaddle/models/issues/1245

WIDERFACE contains some wrong labels. Some w or h< 0. This will result in a data-anchor-sampling error because the number below the square root is less than zero. https://github.com/PaddlePaddle/models/blob/e2131fbc02f86720605aa79fd72720ecce24932a/fluid/face_detection/image_util.py#L147

So an if judgment is added to filter out the unqualified labels.