Fix nvtext::generate_character_ngrams performance regression for longer strings

[davidwendt]

Description

Fixes performance regression when generating character ngrams. The regression was introduced as part of refactoring common code when adding the nvtext::hash_character_ngrams function (Reference #13654). Defactoring the code fixed the regression. Overall, these functions only share about 6 lines of code in common so the defactoring is expected to require minimal maintenance.
The defactoring involves re-instating the original kernel code logic for nvtext::generate_character_ngrams.

Checklist

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

[davidwendt]

Benchmark results

Baseline (before refactor)
--------------------------------------------------------------------------------------------------------
Benchmark                                              Time             CPU   Iterations UserCounters...
--------------------------------------------------------------------------------------------------------
TextNGrams/characters/4096/5/manual_time           0.083 ms        0.102 ms         7867 bytes_per_second=93.6258M/s
TextNGrams/characters/4096/10/manual_time          0.090 ms        0.108 ms         7785 bytes_per_second=215.997M/s
TextNGrams/characters/4096/20/manual_time          0.112 ms        0.130 ms         6190 bytes_per_second=345.847M/s
TextNGrams/characters/4096/40/manual_time          0.148 ms        0.165 ms         4738 bytes_per_second=526.029M/s
TextNGrams/characters/32768/5/manual_time          0.081 ms        0.098 ms         8688 bytes_per_second=768.627M/s
TextNGrams/characters/32768/10/manual_time         0.100 ms        0.116 ms         7041 bytes_per_second=1.51678G/s
TextNGrams/characters/32768/20/manual_time         0.122 ms        0.138 ms         5750 bytes_per_second=2.48205G/s
TextNGrams/characters/32768/40/manual_time         0.176 ms        0.192 ms         3963 bytes_per_second=3.42638G/s
TextNGrams/characters/262144/5/manual_time         0.097 ms        0.113 ms         7243 bytes_per_second=5.00418G/s
TextNGrams/characters/262144/10/manual_time        0.154 ms        0.170 ms         4549 bytes_per_second=7.85904G/s
TextNGrams/characters/262144/20/manual_time        0.296 ms        0.314 ms         2363 bytes_per_second=8.18583G/s
TextNGrams/characters/262144/40/manual_time        0.761 ms        0.779 ms          913 bytes_per_second=6.37766G/s
TextNGrams/characters/2097152/5/manual_time        0.303 ms        0.319 ms         2311 bytes_per_second=12.7862G/s
TextNGrams/characters/2097152/10/manual_time       0.574 ms        0.591 ms         1219 bytes_per_second=16.9356G/s
TextNGrams/characters/2097152/20/manual_time        1.53 ms         1.55 ms          456 bytes_per_second=12.6629G/s
TextNGrams/characters/2097152/40/manual_time        4.90 ms         4.92 ms          143 bytes_per_second=7.92802G/s
TextNGrams/characters/16777216/5/manual_time        1.88 ms         1.89 ms          373 bytes_per_second=16.5321G/s
TextNGrams/characters/16777216/10/manual_time       3.95 ms         3.97 ms          177 bytes_per_second=19.6652G/s
TextNGrams/characters/16777216/20/manual_time       11.7 ms         11.7 ms           60 bytes_per_second=13.3073G/s
TextNGrams/characters/16777216/40/manual_time       37.9 ms         37.9 ms           18 bytes_per_second=8.20249G/s

after refactor

--------------------------------------------------------------------------------------------------------
Benchmark                                              Time             CPU   Iterations UserCounters...
--------------------------------------------------------------------------------------------------------
TextNGrams/characters/4096/5/manual_time           0.084 ms        0.102 ms         7877 bytes_per_second=92.7183M/s
TextNGrams/characters/4096/10/manual_time          0.090 ms        0.108 ms         7737 bytes_per_second=214.974M/s
TextNGrams/characters/4096/20/manual_time          0.113 ms        0.130 ms         6146 bytes_per_second=344.499M/s
TextNGrams/characters/4096/40/manual_time          0.151 ms        0.168 ms         4631 bytes_per_second=514.544M/s
TextNGrams/characters/32768/5/manual_time          0.081 ms        0.098 ms         8573 bytes_per_second=766.268M/s
TextNGrams/characters/32768/10/manual_time         0.099 ms        0.115 ms         7080 bytes_per_second=1.53308G/s
TextNGrams/characters/32768/20/manual_time         0.122 ms        0.138 ms         5764 bytes_per_second=2.48131G/s
TextNGrams/characters/32768/40/manual_time         0.176 ms        0.192 ms         3969 bytes_per_second=3.42508G/s
TextNGrams/characters/262144/5/manual_time         0.095 ms        0.111 ms         7354 bytes_per_second=5.09831G/s
TextNGrams/characters/262144/10/manual_time        0.151 ms        0.167 ms         4626 bytes_per_second=8.02426G/s
TextNGrams/characters/262144/20/manual_time        0.293 ms        0.311 ms         2384 bytes_per_second=8.26735G/s
TextNGrams/characters/262144/40/manual_time         1.06 ms         1.08 ms          657 bytes_per_second=4.5601G/s
TextNGrams/characters/2097152/5/manual_time        0.294 ms        0.310 ms         2385 bytes_per_second=13.1984G/s
TextNGrams/characters/2097152/10/manual_time       0.559 ms        0.576 ms         1251 bytes_per_second=17.4048G/s
TextNGrams/characters/2097152/20/manual_time        1.62 ms         1.63 ms          433 bytes_per_second=12.0312G/s
TextNGrams/characters/2097152/40/manual_time        8.44 ms         8.46 ms           83 bytes_per_second=4.60654G/s
TextNGrams/characters/16777216/5/manual_time        1.80 ms         1.81 ms          389 bytes_per_second=17.2602G/s
TextNGrams/characters/16777216/10/manual_time       3.82 ms         3.84 ms          183 bytes_per_second=20.3582G/s
TextNGrams/characters/16777216/20/manual_time       12.3 ms         12.4 ms           57 bytes_per_second=12.6016G/s
TextNGrams/characters/16777216/40/manual_time       66.4 ms         66.4 ms           11 bytes_per_second=4.68516G/s

The regression was mostly on longer strings (40) with large rows (2097152, 16777216)

Defactoring the code fixed the regression:

--------------------------------------------------------------------------------------------------------
Benchmark                                              Time             CPU   Iterations UserCounters...
--------------------------------------------------------------------------------------------------------
TextNGrams/characters/4096/5/manual_time           0.083 ms        0.101 ms         7922 bytes_per_second=93.8831M/s
TextNGrams/characters/4096/10/manual_time          0.089 ms        0.106 ms         7888 bytes_per_second=219M/s
TextNGrams/characters/4096/20/manual_time          0.110 ms        0.127 ms         6361 bytes_per_second=353.994M/s
TextNGrams/characters/4096/40/manual_time          0.144 ms        0.160 ms         4873 bytes_per_second=541.144M/s
TextNGrams/characters/32768/5/manual_time          0.081 ms        0.099 ms         8584 bytes_per_second=761.802M/s
TextNGrams/characters/32768/10/manual_time         0.098 ms        0.114 ms         7165 bytes_per_second=1.54836G/s
TextNGrams/characters/32768/20/manual_time         0.119 ms        0.135 ms         5876 bytes_per_second=2.53698G/s
TextNGrams/characters/32768/40/manual_time         0.172 ms        0.188 ms         4069 bytes_per_second=3.5083G/s
TextNGrams/characters/262144/5/manual_time         0.095 ms        0.111 ms         7351 bytes_per_second=5.08841G/s
TextNGrams/characters/262144/10/manual_time        0.150 ms        0.165 ms         4671 bytes_per_second=8.10945G/s
TextNGrams/characters/262144/20/manual_time        0.293 ms        0.311 ms         2385 bytes_per_second=8.27545G/s
TextNGrams/characters/262144/40/manual_time        0.738 ms        0.756 ms          949 bytes_per_second=6.57469G/s
TextNGrams/characters/2097152/5/manual_time        0.303 ms        0.318 ms         2313 bytes_per_second=12.8132G/s
TextNGrams/characters/2097152/10/manual_time       0.564 ms        0.581 ms         1238 bytes_per_second=17.24G/s
TextNGrams/characters/2097152/20/manual_time        1.53 ms         1.55 ms          456 bytes_per_second=12.6741G/s
TextNGrams/characters/2097152/40/manual_time        4.75 ms         4.76 ms          147 bytes_per_second=8.1885G/s
TextNGrams/characters/16777216/5/manual_time        1.88 ms         1.89 ms          373 bytes_per_second=16.5346G/s
TextNGrams/characters/16777216/10/manual_time       3.88 ms         3.90 ms          180 bytes_per_second=20.0379G/s
TextNGrams/characters/16777216/20/manual_time       11.7 ms         11.7 ms           60 bytes_per_second=13.3186G/s
TextNGrams/characters/16777216/40/manual_time       36.8 ms         36.8 ms           19 bytes_per_second=8.46076G/s

[davidwendt]

/merge