Add gbenchmark for nvtext ngrams functions

[davidwendt]

Reference #5696
Creates a gbenchmark for nvtext::generate_ngrams() and nvtext::generate_character_ngrams() functions.
The benchmarks measures various string lengths and number of rows.
The nvtext::generate_ngrams() was refactored to use the more efficient make_strings_children which improved its performance by about 50%.

[codecov]

Codecov Report

Merging #7693 (3d384ee) into branch-0.19 (7871e7a) will increase coverage by 0.62%.
The diff coverage is n/a.

❗ Current head 3d384ee differs from pull request most recent head d3155bc. Consider uploading reports for the commit d3155bc to get more accurate results

@@               Coverage Diff               @@
##           branch-0.19    #7693      +/-   ##
===============================================
+ Coverage        81.86%   82.49%   +0.62%     
===============================================
  Files              101      101              
  Lines            16884    17416     +532     
===============================================
+ Hits             13822    14367     +545     
+ Misses            3062     3049      -13     

Impacted Files	Coverage Δ
python/cudf/cudf/core/column/categorical.py	91.97% <ø> (+0.58%)	⬆️
python/cudf/cudf/core/column/column.py	87.86% <ø> (+0.10%)	⬆️
python/cudf/cudf/core/column/datetime.py	89.63% <ø> (+0.54%)	⬆️
python/cudf/cudf/core/column/decimal.py	92.75% <ø> (-2.12%)	⬇️
python/cudf/cudf/core/column/lists.py	92.50% <ø> (+1.10%)	⬆️
python/cudf/cudf/core/column/numerical.py	94.83% <ø> (-0.20%)	⬇️
python/cudf/cudf/core/column/string.py	86.79% <ø> (+0.30%)	⬆️
python/cudf/cudf/core/column/timedelta.py	88.57% <ø> (+0.33%)	⬆️
python/cudf/cudf/core/column_accessor.py	96.01% <ø> (+0.70%)	⬆️
python/cudf/cudf/core/dataframe.py	90.90% <ø> (+0.43%)	⬆️
... and 61 more

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[harrism]

Looks good, just one question about that return std::move

[davidwendt]

@gpucibot merge