'get total number of images, init indices
Parameters
shuffle : bool
whether to shuffle the initial indices'

def _load_image_set_index(self, shuffle):

    self.num_images = 0
    for db in self.imdbs:
        self.num_images += db.num_images
    indices = range(self.num_images)
    if shuffle:
        random.shuffle(indices)
    return indices
