add prototype datasets for MNIST and variants (#4512)

* add prototype datasets for MNIST and variants

* fix mypy

* fix EMNIST labels

* fix code format

* avoid encoding + decoding in every step

* discard data at the binary level instead of after decoding

* cleanup

* fix mypy

Author: pmeier

torchvision/prototype/datasets/_builtin/__init__.py

@@ -1,5 +1,6 @@
 from .caltech import Caltech101, Caltech256
 from .celeba import CelebA
 from .cifar import Cifar10, Cifar100
+from .mnist import MNIST, FashionMNIST, KMNIST, EMNIST, QMNIST
 from .sbd import SBD
 from .voc import VOC