Simpler MLP

[mcabbott]

Same idea as #394, to remove all this baroque junk of making structs you use once, and functions you call once, to just focus on the essentials. But even simpler than #394, no GPU, no NamedTuples, no saving.

• Before: https://github.com/FluxML/model-zoo/blob/8cdc06752598e88c9e4c94e4a594a9bc58a3c128/vision/mlp_mnist/mlp_mnist.jl

• After: https://github.com/FluxML/model-zoo/blob/31e147faef61e7f26dc2d6124afbac0cfe518b6a/vision/mlp_mnist/mlp_mnist.jl

Maybe some of the text in the old one should move to the readme.

[Edit: fixed links to work after merge.]

[ToucheSir]

One benefit of all the extra comments in the original was that it could be rendered as a Literate notebook. I wonder if we should make that official with either a README note or top-level comment with brief instructions.

[ToucheSir]

Running all at once from the terminal won't show any output. Could we use print on the predictions and display the image even if users aren't running from the REPL?

[mcabbott]

For me it does print:

mlp_mnist % /Applications/Julia-1.9.app/Contents/Resources/julia/bin/julia --project mlp_mnist.jl
┌ Info: After epoch = 1
│   loss = 1.8101638024672866
│   train_acc = 75.81
└   test_acc = 76.75
┌ Info: After epoch = 3
│   loss = 0.8674804433248937
│   train_acc = 85.65
└   test_acc = 86.22

It does not show the images, but perhaps that's more intended as a start towards interactive exploration.

[ToucheSir]

If not the images, perhaps we could print out the predictions/ground truth labels in lines 110+115 for comparison.

[mcabbott]

original was that it could be rendered as a Literate notebook

I think quite a few of these files were rendered this way for the old website. But they got stale... and IDK how valuable rendering & commentary is anyway.

This PR and its friend want to go the other direction. Having relatively few, concise, clear examples means you're more likely to find the right one, and it's more likely to be up to date. A separate README for links & more text seems OK, and easier to maintain, less tightly coupled.