'Update grouth truth statitistics.
1. Difficult boxes are ignored when counting the number of ground truth
instances as done in Pascal VOC devkit.
2. Difficult boxes are treated as normal boxes when computing CorLoc related
statitistics.
Args:
groundtruth_class_labels: An integer numpy array of length M,
representing M class labels of object instances in ground truth
groundtruth_is_difficult_list: A boolean numpy array of length M denoting
whether a ground truth box is a difficult instance or not'

def _update_ground_truth_statistics(self, groundtruth_class_labels, groundtruth_is_difficult_list):

    for class_index in range(self.num_class):
        num_gt_instances = np.sum((groundtruth_class_labels[(~ groundtruth_is_difficult_list)] == class_index))
        self.num_gt_instances_per_class[class_index] += num_gt_instances
        if np.any((groundtruth_class_labels == class_index)):
            self.num_gt_imgs_per_class[class_index] += 1
