How to use multiple gpu

[zhenzi0322]

    def build_model(self):
        if self.phase == 'train':
            """ Input Image"""
            img_class = Image_data(self.img_size, self.img_ch, self.dataset_path, self.domain_list, self.augment_flag)
            img_class.preprocess()

            dataset_num = len(img_class.images)
            print("Dataset number : ", dataset_num)

            img_and_domain = tf.data.Dataset.from_tensor_slices((img_class.images, img_class.shuffle_images, img_class.domains))

            gpu_device = '/gpu:1'

            img_and_domain = img_and_domain.shuffle(buffer_size=dataset_num, reshuffle_each_iteration=True).repeat()
            img_and_domain = img_and_domain.map(map_func=img_class.image_processing, num_parallel_calls=AUTOTUNE).batch(self.batch_size, drop_remainder=True)
            img_and_domain = img_and_domain.apply(prefetch_to_device(gpu_device, buffer_size=AUTOTUNE))

How to set up multiple gpu?