Internal promotion rules should distinguish between 0-D scalars and non 0-D scalars

[agoose77]

Version of Awkward Array

main

Description and code to reproduce

The presence of 0-D scalars as input to ak.to_layout presents a tension between preserving scalar-ness and mixing backends between 0D scalars and other arrays.

We could add a rule to promote 0D arrays to 1D-length-1 arrays. But, on balance this would affect typetracer, whose scalars are only ever 0D arrays. In general, the scalar-ness matters. Another solution would be to implement a new function that maps 0D scalars between backends.

[jpivarski]

How related is this to #1057? Is it a duplicate? (Reopen if not.)

[agoose77]

This is a distinct issue, but I'll need to find a reproducer to figure out the best description!

[jpivarski]

Rather than use up numbers, I'll reopen this as a place for you to put the reproducer. Thanks!