Fix segmented-sort to ignore indices outside the offsets

[davidwendt]

Description

Fixes cudf::segmented_sorted_order to ignore indices outside the specified offsets values.

The segmented-sort function in general sorts subsets of the input using a column of offsets (integers) to identify the position of each segment. Here is an example:

input    = { 9, 8, 7, 6, 5, 4, 3, 2, 1, 0 }
offsets1 = { 0,       3,          7,      10 }

There are 3 segments to sort: [0,3), [3,7), and [7,10)
Segment 1 sorts to { 7, 8, 9 }
Segment 2 sorts to { 3, 4, 5, 6 }
Segment 3 sorts to { 0, 1, 2 }
The segmented-sort result is { 7, 8, 9, 3, 4, 5, 6, 0, 1, 2 }

If the offsets do not fully cover all the input the segmented-sort should ignore any segments outside of the offsets.

input    = { 9, 8, 7, 6, 5, 4, 3, 2, 1, 0 }
offsets2 = {          3,          7       }

Here there is only 1 segments to sort: [3,7) => { 3, 4, 5, 6 }
The segmented-sort result is { 9, 8, 7, 3, 4, 5, 6, 2, 1, 0 }
The values before the first offset and after the last offset should be left unchanged.

The gtests have been corrected to expect this behavior.
Also, the SegmentedReductionTestUntyped.PartialSegmentReduction gtest was improved to include offset gaps at the beginning and at the end to verify consistent behavior there as well.

Found while working on #11729

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[codecov]

Codecov Report

Base: 87.40% // Head: 88.11% // Increases project coverage by +0.70% 🎉

Coverage data is based on head (bbf6569) compared to base (f72c4ce).
Patch coverage: 86.42% of modified lines in pull request are covered.

Additional details and impacted files

@@               Coverage Diff                @@
##           branch-22.12   #11888      +/-   ##
================================================
+ Coverage         87.40%   88.11%   +0.70%     
================================================
  Files               133      133              
  Lines             21833    21881      +48     
================================================
+ Hits              19084    19280     +196     
+ Misses             2749     2601     -148     

Impacted Files	Coverage Δ
python/cudf/cudf/core/udf/__init__.py	97.05% <ø> (+47.05%)	⬆️
python/cudf/cudf/io/orc.py	92.94% <ø> (-0.09%)	⬇️
python/cudf/cudf/utils/ioutils.py	79.47% <ø> (ø)
...thon/dask_cudf/dask_cudf/tests/test_distributed.py	18.86% <ø> (+4.94%)	⬆️
python/cudf/cudf/core/_base_index.py	82.20% <43.75%> (-3.35%)	⬇️
python/cudf/cudf/io/text.py	91.66% <66.66%> (-8.34%)	⬇️
python/strings_udf/strings_udf/__init__.py	86.27% <76.00%> (-10.61%)	⬇️
python/cudf/cudf/core/index.py	92.91% <95.16%> (+0.28%)	⬆️
python/cudf/cudf/__init__.py	90.69% <100.00%> (ø)
python/cudf/cudf/core/column/categorical.py	89.34% <100.00%> (ø)
... and 13 more

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[ttnghia]

Most of the time I forgot to ask about doxygen 😞

[davidwendt]

@gpucibot merge