[data request] Add dataset: affNIST

• Name of dataset: affNIST
• URL of dataset: http://www.cs.toronto.edu/~tijmen/affNIST/
• License of dataset: None
• Short description of dataset and use case(s)
  The affNIST dataset for machine learning is based on the well-known MNIST dataset.
  MNIST, however, has become quite a small set, given the power of today's computers, with their multiple CPU's and sometimes GPU's.
  affNIST is made by taking images from MNIST and applying various reasonable affine transformations to them.
  In the process, the images become 40x40 pixels large, with significant translations involved,
  so much of the challenge for the models is to learn that a digit means the same thing in the upper right corner as it does in the lower left corner.

Folks who would also like to see this dataset in tensorflow/datasets, please thumbs-up so the developers can know which requests to prioritize.

And if you'd like to contribute the dataset (thank you!), see our guide to adding a dataset.

Start with docs/add_dataset.md, the repository's guide to adding a dataset, and inspect the affNIST dataset URL in the issue. Done means contributing affNIST to tensorflow/datasets with its stated description, use case, URL, and license information represented as required by the guide.