Per the [Ray docs contributing guide](https://docs.ray.io/en/master/ray-contribute/docs.html), code chunks should be in `.py` files and pulled in via `literalinclude` rather than placed directly in `.rst` files. This PR takes a small step in doing this for the RLlib docs, specifically for the training API doc page.
Note that I had to make some changes to the code itself so that it would run, namely adding missing numpy imports and changing `model.from_batch(...)` to `model(...)` in a couple places.
Co-authored-by: Max Pumperla <max.pumperla@googlemail.com>