Add example program of MNIST using pytorch DDP

[KWang1998]

MNIST Pytorch program is obtained from Pytorch/MNIST
A code patch mnist.patch must be applied first.

Command make check automatically downloads, applies the patch,
and run the training with small settings, i.e.

• train batch size = 4,
• test batch size = 2,
• number of epochs = 3,
• number of training samples = 60, and
• number of testing samples = 12