add check for cells with imports and code

When building the docs nbdev runs all cells containing import statements, so it's in our best interest to isolate the imports in separate cells so that other code like tests isn't run when building the docs. This adds a check to pre-commit that will fail if there are such cells and will print the notebook path which contains them.

These are mostly changes to the notebooks, however in two modules we were running the following:

logging.getLogger("pytorch_lightning").setLevel(logging.ERROR)
warnings.filterwarnings("ignore")

which I'm guessing was unintended and so this deletes that.

Nixtla / neuralforecast

add check for cells with imports and code #954