find out which indexes correspond to given image set (train or val)

        Parameters:
        ----------
        shuffle : boolean
            whether to shuffle the image list
        Returns:
        ----------
        entire list of images specified in the setting

def _load_image_set_index(self, shuffle):
        """
        find out which indexes correspond to given image set (train or val)

        Parameters:
        ----------
        shuffle : boolean
            whether to shuffle the image list
        Returns:
        ----------
        entire list of images specified in the setting
        """
        image_set_index_file = os.path.join(self.data_path, 'ImageSets', 'Main', self.image_set + '.txt')
        assert os.path.exists(image_set_index_file), 'Path does not exist: {}'.format(image_set_index_file)
        with open(image_set_index_file) as f:
            image_set_index = [x.strip() for x in f.readlines()]
        if shuffle:
            np.random.shuffle(image_set_index)
        return image_set_index