Deprecate symbolic Join/Split axis

[ricardoV94]

Description

This make working with these Ops much harder because we always have to check whether the axis is constant (and we can reason about the operation) or not (and we usually can't do anything).

I would force axis to be static. If users want a symbolic axis they can always implement their own Op for that purpose.

I don't see a clean way of gradually deprecating the symbolic axis argument so I would do it as a major change and get it done with.