add mnist

[rtqichen]

Adds MNIST dataset. Seems to be working, at least for my own experiments. Resolves #13.

import torchvision.datasets as dset
training_data = dset.MNIST('data/mnist', train=True, transform=transforms.ToTensor(), download=True)
training_loader = torch.utils.data.DataLoader(training_data, batch_size=32, shuffle=True, num_workers=4)

By default, returns a grayscale PIL Image (mode L). Works with the ToTensor() transform and DataLoader.

[apaszke]

@soumith @colesbury how about not returning PIL images from this one? I don't think we always need to return the same type

[apaszke]

Thanks!