'given image index, find out annotation path
Parameters:
index: int
index of a specific image
Returns:
full path of annotation file'

def _label_path_from_index(self, index):

    label_file = os.path.join(self.label_dir, (index + self.label_extension))
    assert os.path.exists(label_file), 'Path    does    not    exist:    {}'.format(label_file)
    return label_file
