Rename step() to model_step() for compatibility with recent lightning release

[ashleve]

What does this PR do?

Renames model.step() to model.model_step() for compatibility with recent lightning release.

(lightning is now using step() as its own internal function, so we can't override it)

Before submitting

• Did you make sure title is self-explanatory and the description concisely explains the PR?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you list all the breaking changes introduced by this pull request?
• Did you test your PR locally with pytest command?
• Did you run pre-commit hooks with pre-commit run -a command?

Did you have fun?

Make sure you had fun coding 🙃