Add ignore regions to reduce effect from synonymous objects

some datasets like CrowdHuman contains **'ignore regions'** bounding boxes annotation, there are 100k ignore bounding boxes in CrowdHuman, when we train a detector based on this dataset, we may want to reduce the effect from human doll, human billboard or dense crowd human. these part of regions should not consider as positive and negative supervise, bug I notice Detectron2 don't have a feature to process 'ignore' annotation during dataloader and matcher, this feature cound be used for lots of projects which need ignore labels