Fix public export promote_nullable

[EmilyMohellenbrandQC]

Bug

promote_nullable is publicly exposed, but it's not working because it is not imported properly in the init file.

Changes

Add import promote_nullable in ndonnx.__init__

[cbourjau]

I'm not particularly happy with this function; being explicit about the mask as make_nullable is doing seems better. For the sake of stability, we may make an exercise out of this though: Let's fix the export but deprecate it immediately in favor of make_nullable. Then we remove it in a release or two from the public interface.

[adityagoel4512]

I agree - there isn't a very natural way to make use of this function without messing with an array's fields directly. Can you please add a DeprecationWarning @EmilyMohellenbrandQC?

Added deprecation warning and TODO for removal

[cbourjau]

When deprecating a function we should make sure that we don't use it ourselves in this code base, too. Otherwise, we thrust deprecation warnings into the user's face that they can do nothing about. Unfortunately, this happened here: https://github.com/Quantco/ndonnx/actions/runs/9892611298/job/27325701365#step:5:41