add prototype imagenet dataset

torchvision/prototype/datasets/_builtin/__init__.py

@@ -2,6 +2,7 @@
 from .celeba import CelebA
 from .cifar import Cifar10, Cifar100
 from .coco import Coco
+from .imagenet import ImageNet
 from .mnist import MNIST, FashionMNIST, KMNIST, EMNIST, QMNIST
 from .sbd import SBD
 from .voc import VOC