Accept const refs instead of const unique_ptr refs in reduce and scan APIs.

[vyasr]

Description

There is almost never a good reason to pass arguments as unique_ptr<T> const&. Since those arguments cannot be modified, the only use case is accessing the underlying pointer, at which point the function better communicates its intent by accepting the underlying pointer/reference as an argument instead and is also more flexible as a result.

Resolves #10393

Checklist

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

[vyasr]

For whoever ends up reviewing this for the Java side (perhaps @jlowe) I don't think this breaking change has any impact on the Spark plugin, it only affects the JNI and I have made the necessary changes.

[bdice]

Approving C++ and Cython changes.

[davidwendt]

Can you add a real description to this PR? It will be added to the changelog at the end of the release.

[bdice]

Can you add a real description to this PR? It will be added to the changelog at the end of the release.

Only the PR title is added to the changelog. The description is used for the merge commit message. (Still needs a real description.)

[codecov]

Codecov Report

Base: 87.40% // Head: 86.88% // Decreases project coverage by -0.52% ⚠️

Coverage data is based on head (16472ae) compared to base (f72c4ce).
Patch coverage: 82.35% of modified lines in pull request are covered.

❗ Current head 16472ae differs from pull request most recent head 97070bd. Consider uploading reports for the commit 97070bd to get more accurate results

Additional details and impacted files

@@               Coverage Diff                @@
##           branch-22.12   #11960      +/-   ##
================================================
- Coverage         87.40%   86.88%   -0.53%     
================================================
  Files               133      133              
  Lines             21833    21982     +149     
================================================
+ Hits              19084    19099      +15     
- Misses             2749     2883     +134     

Impacted Files	Coverage Δ
python/cudf/cudf/core/dataframe.py	93.73% <ø> (-0.05%)	⬇️
python/cudf/cudf/core/frame.py	93.68% <ø> (ø)
python/cudf/cudf/core/index.py	92.96% <ø> (+0.33%)	⬆️
python/cudf/cudf/core/indexed_frame.py	92.03% <ø> (ø)
python/cudf/cudf/core/scalar.py	90.52% <ø> (+1.25%)	⬆️
python/cudf/cudf/core/series.py	95.78% <ø> (+0.03%)	⬆️
python/cudf/cudf/core/udf/__init__.py	50.00% <ø> (ø)
python/cudf/cudf/io/orc.py	92.94% <ø> (-0.09%)	⬇️
python/cudf/cudf/io/text.py	91.66% <ø> (-8.34%)	⬇️
python/cudf/cudf/testing/dataset_generator.py	72.83% <ø> (-0.42%)	⬇️
... and 49 more

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[vyasr]

Added a description

[vyasr]

@gpucibot merge