Download and extract the tarball from Alex Krizhevsky's website.

def maybe_download_and_extract(self):
        """Download and extract the tarball from Alex Krizhevsky's website."""
        if not os.path.exists(self.cifar10_dir):

            if not os.path.exists(self.data_dir):
                os.makedirs(self.data_dir)

            def _progress(count, block_size, total_size):
                status_msg = '\r>> Downloading {} {:>3}%   '
                sys.stdout.write(status_msg.format(self.cifar10_tarball, float(count * block_size) / total_size * 100.0))
                sys.stdout.flush()

            file_path, _ = urlretrieve(CIFAR10_URL, self.cifar10_tarball, _progress)

            stat_info = os.stat(file_path)
            print('\nSuccessfully downloaded', file_path, stat_info.st_size, 'bytes.\n')

            tarfile.open(file_path, 'r:gz').extractall(self.data_dir)