Change cudf::detail::tdigest to cudf::tdigest::detail

[davidwendt]

Description

Changes cudf::detail::tdigest to cudf::tdigest::detail in the tdigest source files.
While working on #12049, found there was a mixture of cudf::tdigest and cudf::detail::tdigest that seemed confusing and inconsistent. Changing to cudf::tdigest::detail made this code easier to follow.
Also, move the size_begin() member function in tdigest_column_view out as a standalone function in a separate .cuh header since it is only used in a few places and the tdigest_column_view.cuh is included in many places. This allowed changing the tdigest_column_view.cuh to a .hpp file.

Depends on #12049

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: 87.47% // Head: 88.07% // Increases project coverage by +0.59% 🎉

Coverage data is based on head (83bb91a) compared to base (f817d96).
Patch has no changes to coverable lines.

Additional details and impacted files

@@               Coverage Diff                @@
##           branch-22.12   #12050      +/-   ##
================================================
+ Coverage         87.47%   88.07%   +0.59%     
================================================
  Files               133      135       +2     
  Lines             21826    22025     +199     
================================================
+ Hits              19093    19398     +305     
+ Misses             2733     2627     -106     

Impacted Files	Coverage Δ
python/cudf/cudf/core/column/interval.py	85.45% <0.00%> (-9.10%)	⬇️
python/cudf/cudf/io/text.py	91.66% <0.00%> (-8.34%)	⬇️
python/cudf/cudf/core/_base_index.py	81.28% <0.00%> (-4.27%)	⬇️
python/cudf/cudf/io/json.py	92.06% <0.00%> (-2.68%)	⬇️
python/cudf/cudf/utils/utils.py	89.91% <0.00%> (-0.69%)	⬇️
python/cudf/cudf/core/column/timedelta.py	90.17% <0.00%> (-0.58%)	⬇️
python/cudf/cudf/core/column/datetime.py	89.21% <0.00%> (-0.51%)	⬇️
python/cudf/cudf/core/column/column.py	87.96% <0.00%> (-0.46%)	⬇️
python/dask_cudf/dask_cudf/core.py	73.72% <0.00%> (-0.41%)	⬇️
python/cudf/cudf/io/parquet.py	90.45% <0.00%> (-0.39%)	⬇️
... and 39 more

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[ttnghia]

This is a potential improvement with documentation: We need to standardize either cudf::detail::module_name or cudf::module_name::detail namespace. From this PR, cudf::module_name::detail should be favored and published into the dev guide.

[bdice]

This is a potential improvement with documentation: We need to standardize either cudf::detail::module_name or cudf::module_name::detail namespace. From this PR, cudf::module_name::detail should be favored and published into the dev guide.

This also came up in #12046 (comment).

[vuule]

This is a potential improvement with documentation: We need to standardize either cudf::detail::module_name or cudf::module_name::detail namespace. From this PR, cudf::module_name::detail should be favored and published into the dev guide.

This also came up in #12046 (comment).

I'm planning to open an issue for cuIO (which is mostly in the wrong order of namespaces). Let me know if this is something we need to fix in the rest of libcudf as well, so the issue reflects that too.

[davidwendt]

@gpucibot merge