[FEA] Generate offsets from element labels

[ttnghia]

Given an array of integer values which may be the labels of some list elements, we want to generate an array of offsets so we can create a lists column from these offsets and elements gathered from another lists column using the input labels as markup for gather map.

For example:

input_labels = [0, 0, 0, 0, 1, 1, 2, 2, 2, 2, 5, 5]
output_offsets = [0, 4, 6, 10, 12]

This is basically extracting the existing function from drop_list_duplicates (

cudf/cpp/src/lists/drop_list_duplicates.cu

Line 486 in ca952f8

std::unique_ptr<column> generate_output_offsets(size_type num_lists,

). As such, it will be continuingly used in drop_list_duplicates. In addition, it should be used to implement the set-like operations (#10409).

[ttnghia]

Note that this is basically the opposite operation of the previous issue (#10905) filed for unblocking the same FEA.

[ttnghia]

In addition, the input labels may be discontinued (like [0, 0, 2, 2]) due to null/empty lists and we want to generate a full offsets array ([0, 2, 2, 4]).