Plans that have multiple output partitions at the top level fail

I have a physical plan that looks like this when it enters ray
```
[ output_partitions: 16] ProjectionExec: expr=[...]
[ output_partitions: 16] RayStageExec[0] (output_partitioning=RoundRobinBatch(16))
[ output_partitions: 16] RepartitionExec: partitioning=RoundRobinBatch(16), input_partitions=1
[ output_partitions: 1] GlobalLimitExec: skip=0, fetch=1
[ output_partitions: 1] CoalescePartitionsExec
[ output_partitions: 16] CoalesceBatchesExec: target_batch_size=8192, fetch=1
[ output_partitions: 16] FilterExec: timestamp@0 >= 1736899200000000 AND timestamp@0 < 1736985600000000
[ output_partitions: 16] DeltaScan
[ output_partitions: 16] ParquetExec: file_groups={...]
```
In this case, Ray will add a RayStageExec on top of the ProjectionExec, and then fail because `Last stage expected to have one partition`

I managed to fix it locally by ensuring that the plan has 1 output partition
```
let num_output_partitions = match plan.output_partitioning() {
Partitioning::RoundRobinBatch(x) => x,
Partitioning::Hash(_, x) => x,
Partitioning::UnknownPartitioning(x) => x
};
if *num_output_partitions > 1 {
plan = Arc::new(RepartitionExec::try_new(plan, Partitioning::RoundRobinBatch(1))?);
}

```