Add PyTorch example

[seanpmorgan]

It'd be nice to have an example for the demonstration

[davidslater]

For examples, I was thinking that we could do a simple network transformation defense. We could take a pretrained resnet and replace the ReLU activations with ReLU6 (or similar hardtanh) with some max value (ostensibly, to prevent adversarial examples from generating very large activations).

Should be reasonbly straightforward, similar to: https://gist.github.com/chauhan-utk/40d25e60cfd6d2a393e34cfcb7eb25f6