Add scalable model training tutorials

[duckontheweb]

Adds tutorials for scalable model training using Radiant MLHub datasets.

Also adds flake8 configuration to avoid conflict between black and flake8.

[duckontheweb]

@KennSmithDS @DanielNwaeze I had to make some changes to remove unused imports and fix a few other lint issues. I haven't tried running the new versions of these on the PC Hub, so you may want to give them 1 more run to make sure none of the changes affected runtime behavior.

@DanielNwaeze I will also flag you with some specific lint/runtime issues that I could not resolve myself.

[duckontheweb]

@TomAugspurger this is ready for review.

cc: @DanielNwaeze @KennSmithDS