Is GetOrAdd safe for varying partition sizes within a single driver stage?

[pgrete]

Changing the partition sizes within a driver is generally allowed (currently), but it's unclear of that change is properly reflected in the caching label mechanism.
We/I should double check of there are potential issue as there are/might be places in the code where we implicitly assume a single pack (e.g.. for guaranteed non-blocking reductions across all blocks of a rank).

[Yurlungur]

This may also be different between sparse and legacy packs (another reason to move to a single codepath at some point)

[pgrete]

And for the GMG GetOrAdd, see #911 (comment)