[FEA][AUDIT][SPARK-52921][SQL] Specify outputPartitioning for UnionExec for same output partitoning as children operators

[abellina]

This is a spark 4.1 audit task.

UnionExec supports outputPartitioning, when children have the same output partitioning. I am not 100% sure why we would want this, but filing in case we need it. According to the issue, Union has "unknown" partitioning otherwise. My guess is that if we don't follow suit, spark-rapids could add shuffles when the CPU case wouldn't. That said, I am not 100% sure, and wanted to get some comments from @revans2 and others.

There is a follow up I was looking at for audit (apache/spark@c0acf45023f), so I created the issue for the main spark issue to discuss.

Here's another related follow up: apache/spark@8edc7685b97

[revans2]

We kind of do need this, but it might just be a nice to have. Spark uses the output partitioning and the required partitioning to decide if it needs to insert in a shuffle to make the partitioning what it needs. I think all of that has been planned out before us already, but it would be good to try and match it just in case. Especially because Union is a logical operation only. We don't do anything on the GPU to make the union happen.