Rework cudf::replace_nulls to use strings::detail::copy_if_else

[davidwendt]

Description

Removes the specialized kernels for strings in cudf::replace_nulls and replaces them with a call to cudf::strings::detail::copy_if_else which is already enabled with offsetalator support and optimized for long strings.
This will also allow cudf::replace_nulls to use large strings with no further changes.
Also includes a replace_nulls benchmark for strings.

Checklist

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

[ttnghia]

Can you post some benchmark number comparing before vs after this change please?

[davidwendt]

Performance benchmark for this change.

NVIDIA RTX A6000

| row_width |  num_rows |   Ref Time |   Cmp Time |           Diff |   %Diff |
|-----------|-----------|------------|------------|----------------|---------|
|    32     |    32768  |  72.199 us |  94.023 us |      21.825 us |  30.23% |
|    64     |    32768  |  86.534 us | 111.229 us |      24.695 us |  28.54% |
|    128    |    32768  | 127.262 us | 149.321 us |      22.059 us |  17.33% |
|    256    |    32768  | 226.015 us |  90.125 us |    -135.890 us | -60.12% |
|    512    |    32768  | 489.888 us | 101.352 us |    -388.537 us | -79.31% |
|   1024    |    32768  |   1.103 ms | 126.929 us |    -976.487 us | -88.50% |
|   2048    |    32768  |   2.360 ms | 176.211 us |   -2183.316 us | -92.53% |
|    32     |   262144  | 112.676 us | 164.440 us |      51.765 us |  45.94% |
|    64     |   262144  | 221.317 us | 304.757 us |      83.440 us |  37.70% |
|    128    |   262144  |   1.706 ms |   1.907 ms |     201.129 us |  11.79% |
|    256    |   262144  |   5.409 ms | 220.857 us |   -5188.351 us | -95.92% |
|    512    |   262144  |  14.826 ms | 303.993 us |  -14521.897 us | -97.95% |
|   1024    |   262144  |  35.427 ms | 499.784 us |  -34927.009 us | -98.59% |
|   2048    |   262144  |  80.428 ms | 891.051 us |  -79536.536 us | -98.89% |
|    32     |  2097152  | 461.374 us | 638.544 us |     177.169 us |  38.40% |
|    64     |  2097152  |   1.243 ms |   1.463 ms |     220.090 us |  17.70% |
|    128    |  2097152  |  10.630 ms |  13.658 ms |       3.028 ms |  28.49% |
|    256    |  2097152  |  44.534 ms |   1.178 ms |  -43356.375 us | -97.36% |
|    512    |  2097152  | 121.787 ms |   1.894 ms | -119892.600 us | -98.44% |
|    32     | 16777216  |   3.295 ms |   4.492 ms |       1.198 ms |  36.35% |
|    64     | 16777216  |   9.511 ms |  11.068 ms |       1.556 ms |  16.37% |

This result is mainly from the gather logic now.
Mostly improvements on long strings -- some close to 90x.
Shorter strings are a bit slower by a few milliseconds.

[bdice]

This looks great to me. Very nice performance improvements, and the kernels are simpler.

[davidwendt]

/merge