Include <optional> in multibyte split.

[bdice]

#10150 broke compiler support for GCC 11 (built locally) because it was missing #include <optional> in a couple files. This fixes it. cc: @cwharris

[codecov]

Codecov Report

Merging #10385 (c06a778) into branch-22.04 (a7d88cd) will decrease coverage by 0.56%.
The diff coverage is n/a.

@@               Coverage Diff                @@
##           branch-22.04   #10385      +/-   ##
================================================
- Coverage         10.42%    9.85%   -0.57%     
================================================
  Files               119      126       +7     
  Lines             20603    22606    +2003     
================================================
+ Hits               2148     2228      +80     
- Misses            18455    20378    +1923     

Impacted Files	Coverage Δ
...ython/custreamz/custreamz/tests/test_dataframes.py	99.39% <0.00%> (-0.01%)	⬇️
python/cudf/cudf/errors.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/orc.py	0.00% <0.00%> (ø)
python/cudf/cudf/_version.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/ops.py	0.00% <0.00%> (ø)
python/cudf/cudf/datasets.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/frame.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/index.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/parquet.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/scalar.py	0.00% <0.00%> (ø)
... and 45 more

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[karthikeyann]

rerun tests

[bdice]

@gpucibot merge