Supervised Learning Training Loop

We've provided a simple SL training loop in sl_loop.py, which avoids using our dataset classes and instead defines the data loading in a more self-contained way. This is for people who like to write their own training loops or learn about how things work under the hood. Our more performant implementation in supervised/train.py does basically the same thing, but with some performance optimizations, and with some additional features like periodic evals.