Args:
                rel_name: relative path to where donwloaded nnp is saved.
                rel_url: relative url path to where nnp is downloaded from.

def _load_nnp(self, rel_name, rel_url):
        '''
            Args:
                rel_name: relative path to where donwloaded nnp is saved.
                rel_url: relative url path to where nnp is downloaded from.

            '''
        from nnabla.utils.download import download
        path_nnp = os.path.join(
                get_model_home(), 'imagenet/{}'.format(rel_name))
        url = os.path.join(get_model_url_base(),
                           'imagenet/{}'.format(rel_url))
        logger.info('Downloading {} from {}'.format(rel_name, url))
        dir_nnp = os.path.dirname(path_nnp)
        if not os.path.isdir(dir_nnp):
            os.makedirs(dir_nnp)
        download(url, path_nnp, open_file=False, allow_overwrite=False)
        print('Loading {}.'.format(path_nnp))
        self.nnp = NnpLoader(path_nnp)