Add the MNIST example from MLJFlux/examples

[ablaom]

This includes a demonstration of wrapping a model in IteratedModel which would be new here.

[adityasaini70]

@ablaom Are you talking about the notebook at https://github.com/FluxML/MLJFlux.jl/blob/dev/examples/mnist/mnist.ipynb? This will be a really great addition to the current arsenal, and I'd love to work on this! Can you assign this to me?

[ablaom]

Thanks for offer of help! Yes, I mean this one. But I'll let @tlienart handle assignment. As there is already a Literate-compatible script there (the .jl file) there's not much to actually do here. And it might make sense to make the actual PR after @tlienart finishes his upgrades.