get total number of images, init indices

        Parameters
        ----------
        shuffle : bool
            whether to shuffle the initial indices

def _load_image_set_index(self, shuffle):
        """
        get total number of images, init indices

        Parameters
        ----------
        shuffle : bool
            whether to shuffle the initial indices
        """
        self.num_images = 0
        for db in self.imdbs:
            self.num_images += db.num_images
        indices = list(range(self.num_images))
        if shuffle:
            random.shuffle(indices)
        return indices