[fbsync] add prototype imagenet dataset (#4640)

Summary:
* add prototype imagenet dataset

* add missing checksums

* fix mypy

* add human readable categories

* cleanup

* sort categories ascending based on wnid

* remove accidentally added file

* cleanup category file generation

* fix mypy

Reviewed By: NicolasHug

Differential Revision: D31916331

fbshipit-source-id: 38a598f951923342e488f0188f40c74d5b13108c

Author: datumbox

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