[BUG] Missing numRows in the ColumnarBatch created in GpuBringBackToHost

[jihoonson]

Describe the bug
GpuBringBackToHost creates the ColumnarBatch without numRows, which will be set to 0 in this case. This can confuse any logic relying on the numRows metric during the query processing, such as adaptive query optimizer. The ColumnarBatch returned would be always regarded as if it were an empty batch.

Expected behavior
The right number of rows should be retrieved from the hostColumns and then set for the ColumnarBatch before it is returned.

[revans2]

@jihoonson great catch. Could you also check other places that we create a ColumnarBatch to be sure there are no other issues like this? The API is kind of bad because you can create a ColumnarBatch and then set the number of rows afterwards.

[jihoonson]

@revans2 I have checked other places, and did not see any. I agree that this API does not have the best design. I wonder if we should do something about it, such as adding safe wrappers, e.g., rapids.ColumnarBatches.empty() and rapids.ColumnarBatches.create(ColumnVector[] columns, int numRows), and forbidding unsafe APIs.

[jihoonson]

Regarding forbidding the use of unsafe APIs, I meant that we could use the forbidden API checker to catch them during the build.

[jihoonson]

Filed #11369.