preprocess all ground-truths

        Returns:
        ----------
        labels packed in [num_images x max_num_objects x 5] tensor

def _load_image_labels(self):
        """
        preprocess all ground-truths

        Returns:
        ----------
        labels packed in [num_images x max_num_objects x 5] tensor
        """
        temp = []

        # load ground-truths
        for idx in self.image_set_index:
            label_file = self._label_path_from_index(idx)
            with open(label_file, 'r') as f:
                label = []
                for line in f.readlines():
                    temp_label = line.strip().split()
                    assert len(temp_label) == 5, "Invalid label file" + label_file
                    cls_id = int(temp_label[0])
                    x = float(temp_label[1])
                    y = float(temp_label[2])
                    half_width = float(temp_label[3]) / 2
                    half_height = float(temp_label[4]) / 2
                    xmin = x - half_width
                    ymin = y - half_height
                    xmax = x + half_width
                    ymax = y + half_height
                    label.append([cls_id, xmin, ymin, xmax, ymax])
                temp.append(np.array(label))
        return temp