Add ngroup

[shwina]

Description

Adds the GroupBy.ngroup() method. Closes #11848

Checklist

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

[shwina]

The changes to this function are tangential, but I discovered the need for a docstring here, and a bug that would lead to a segfault if values was empty.

[codecov]

Codecov Report

Base: 87.46% // Head: 87.49% // Increases project coverage by +0.02% 🎉

Coverage data is based on head (dbcf905) compared to base (3f9b3fe).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@               Coverage Diff                @@
##           branch-22.12   #11871      +/-   ##
================================================
+ Coverage         87.46%   87.49%   +0.02%     
================================================
  Files               133      133              
  Lines             21792    21880      +88     
================================================
+ Hits              19061    19144      +83     
- Misses             2731     2736       +5     

Impacted Files	Coverage Δ
python/cudf/cudf/core/groupby/groupby.py	91.51% <100.00%> (+0.50%)	⬆️
python/strings_udf/strings_udf/__init__.py	86.27% <0.00%> (-8.73%)	⬇️
python/cudf/cudf/io/text.py	91.66% <0.00%> (-8.34%)	⬇️
python/cudf/cudf/core/_base_index.py	82.20% <0.00%> (-3.35%)	⬇️
python/cudf/cudf/core/udf/__init__.py	50.00% <0.00%> (-1.62%)	⬇️
python/cudf/cudf/io/orc.py	92.94% <0.00%> (-0.09%)	⬇️
python/cudf/cudf/utils/ioutils.py	79.47% <0.00%> (ø)
python/cudf/cudf/core/dataframe.py	93.77% <0.00%> (+0.04%)	⬆️
python/cudf/cudf/core/column/string.py	88.55% <0.00%> (+0.12%)	⬆️
... and 4 more

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[galipremsagar]

Could we also add ngroup entry to Computations / descriptive stats group in https://github.com/rapidsai/cudf/blob/branch-22.12/docs/cudf/source/api_docs/groupby.rst

[galipremsagar]

@gpucibot merge