Add gbenchmark for nvtext tokenize functions

[davidwendt]

Reference #5696
Creates gbenchmarks for nvtext::tokenize(), nvtext::count_tokens() and nvtext::ngrams_tokenize() functions.
The benchmarks measures various string lengths and number of rows.

These functions use the make_strings_column factory optimized in #7576

[codereport]

lgtm

[codecov]

Codecov Report

Merging #7684 (8bf0c22) into branch-0.19 (7871e7a) will increase coverage by 0.61%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           branch-0.19    #7684      +/-   ##
===============================================
+ Coverage        81.86%   82.47%   +0.61%     
===============================================
  Files              101      101              
  Lines            16884    17402     +518     
===============================================
+ Hits             13822    14353     +531     
+ 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	95.45% <ø> (+0.14%)	⬆️
python/cudf/cudf/core/dataframe.py	90.90% <ø> (+0.44%)	⬆️
... and 64 more

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[codereport]

@gpucibot merge