Created by: heavengate

fix class should be valid if gt > 0 and true positive predction = 0 In ppdet/utils/map_utils.py, if gt number > 0 and true_positive prediction number = 0, this class will be skipped, but it should be calculated as a valid class, this bug may make mAP higher. Normally, like in Pascal VOC dataset, true_positive predictions for a class would not be 0, this bug has no influence on released models currently