[FEA] Add an internal utility API to return an offsets column of a sliced column starting with zero

[ttnghia]

For nested sliced columns that have an offsets column child, their offsets columns may contain values that do not start from zero. For example:

offsets = [5, 7, 20, ...]

Many operations on these sliced columns need to generate an output offsets column that starts with zero. For example, with the input column having offsets given above, the output offsets column should be:

offsets = [0, 2, 13, ...]

Such output offsets column is generated simply by subtracting all the values with the first value. Yes, very simple.

I would like to have an internal API implementing this feature. Currently, there are several other APIs using it by implementing private code in their .cu files. For example:

• lists/segmented_sort.cu
• lists/drop_list_duplicates (FYI: Add struct type support for drop_list_duplicates #9202 (comment))

[jrhemstad]

Are there situations where we could instead create a transform iterator to materialize the "clean" offsets instead of needing to materialize the new offsets?

[ttnghia]

I believe the answer is yes. However, in the two files listed above, a new offsets column is needed to generating an output lists column (using make_lists_column).

[jrhemstad]

Maybe the solution here can then be to add that iterator and then provide a convenience function that just uses that iterator to fill a new column.

[github-actions]

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