COLLECT_LIST support returning empty output columns.

[mythrocks]

Fixes the group-by portion of #7611.

When COLLECT_LIST() or COLLECT_SET() aggregations are called on a grouped input, if the input column is empty, then one sees the following failure:

C++ exception with description "cuDF failure at: .../cpp/src/column/column_factories.cpp:67: 
make_empty_column is invalid to call on nested types" thrown in the test body.

The operation should have resulted in an empty LIST column. make_empty_column() does not support LIST types (in part because the data_type parameter does not capture the types of the child columns).

This commit fixes this by constructing the output column from the specified values input, but only for COLLECT_LIST() and COLLECT_SET(); other aggregation types are unchanged.

[codecov]

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[codereport]

lgtm

[mythrocks]

gpuCI/cudf/build/python — Build failure/error

I'm running in circles through the CI links, trying to figure out what the failure is. It doesn't seem obvious. I'll re-kick it in time for the second code review.

[ttnghia]

Currently, we have this factory function to make an empty column in column_factories.*:

/**
 * @brief Creates an empty column of the specified @p type
 *
 * An empty column contains zero elements and no validity mask.
 *
 * @param[in] type The column data type
 * @return Empty column with desired type
 */
std::unique_ptr<column> make_empty_column(data_type type);

I would like to have a similar function for other types as well, putting at the same place with the existing function (column_factories.*). Because we need to know type of children columns for lists/structs column, I would suggest to have these new functions (or something better):

template<class ChildTypes>
std::unique_ptr<column> make_empty_column(data_type type, ChildTypes const& child_types);

template<>
std::unique_ptr<column> make_empty_column<std::vector<type>>(data_type type, std::vector<type> const& child_types);

So, for constructing an empty column of nested type, just pass in the additional array of child types. That array can be an array of array of array of... of cudf type. The first function above recursively calls itself. The second is the base case.

Having the factory APIs above, we can use it elsewhere in many other places.

PS 1: The proposed functions above are not good enough as they cannot handle uneven nested levels. We can use a customized struct instead of std::vector.

PS 2: Maybe this is out of scope of this PR. We can make it a new PR for the next release instead.

[mythrocks]

We can use a customized struct instead of std::vector...

I was about to mention that nesting need not be even, till I read this part.

I know there have been prior discussions on conveying nested type information. I think the consensus was to populate child columns all the way down , and use exemplars when constructing empties (a la empty_like()).

PS 2: Maybe this is out of scope of this PR.

Thanks, I concur. :/

[ttnghia]

Reference: #8178

[mythrocks]

@gpucibot merge