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

def load_image_set_index(self):

    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()]
    return image_set_index
