Set numRows for the ColumnBatch created in GpuBringBackToHost

[jihoonson]

Fixes #11364.

[jihoonson]

build

[jlowe]

This looks good, but a quick grep of the source shows there's some other cases where this is done incorrectly, e.g.:

• TestingV2Source
• CurrentBatchIterator
• GpuBroadcastNestedLoopJoinExecBase (empty batch, probably OK)
• GpuPartitionerExpr
• GpuBloomFilterMightContain (empty batch, probably OK)
• ImplicitTestsSuite
• RapidsShuffleTestHelper

If not handling here, we should track these for followup.

[jihoonson]

@jlowe I did have checked all other places except for those in the unit test assuming they are fine. AFAIT, those places in production that use the constructor without the row count have two patterns.

1. They create an empty ColumnBatch. The row count is defaulted to 0, which is legit in this case.
2. They set the row count after the ColumnBatch is created, such as in CurrentBatchIterator. I think it's better to use a "safer" constructor that explicitly takes the row count, but it is not a bug.

I think the real issue here is that the ColumnBatch constructor API is not safe as it is easy for human to make mistakes. An approach to solve that is forbidding the use of those unsafe APIs and adding safer wrappers around them as I proposed in here.

[jihoonson]

build

[jihoonson]

I think the real issue here is that the ColumnBatch constructor API is not safe as it is easy for human to make mistakes. An approach to solve that is forbidding the use of those unsafe APIs and adding safer wrappers around them as I proposed in here.

Filed the suggestion in #11369.