fixing bug with multi-threaded access for variable length string arrays for onnx models.

[michaelgsharp]

Fixes bug where multiple threads would change the same variable resulting in corrupted values.

[codecov]

Codecov Report

Merging #6537 (96055c5) into main (c55038f) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6537      +/-   ##
==========================================
- Coverage   68.46%   68.44%   -0.02%     
==========================================
  Files        1173     1174       +1     
  Lines      247651   248037     +386     
  Branches    25846    25909      +63     
==========================================
+ Hits       169547   169766     +219     
- Misses      71375    71517     +142     
- Partials     6729     6754      +25     

Flag	Coverage Δ
Debug	68.44% <100.00%> (-0.02%)	⬇️
production	62.82% <100.00%> (-0.03%)	⬇️
test	88.93% <ø> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
src/Microsoft.ML.OnnxTransformer/OnnxTransform.cs	89.83% <100.00%> (ø)
...c/Microsoft.ML.FastTree/Utils/ThreadTaskManager.cs	79.48% <0.00%> (-20.52%)	⬇️
src/Microsoft.Data.Analysis/IDataView.Extension.cs	91.72% <0.00%> (-4.20%)	⬇️
src/Microsoft.ML.Maml/MAML.cs	25.38% <0.00%> (-1.53%)	⬇️
src/Microsoft.Data.Analysis/Strings.Designer.cs	42.36% <0.00%> (-0.91%)	⬇️
src/Microsoft.ML.DataView/VBuffer.cs	94.44% <0.00%> (ø)
...oft.Data.Analysis.Tests/DataFrameIDataViewTests.cs	100.00% <0.00%> (ø)
.../Microsoft.Data.Analysis/VBufferDataFrameColumn.cs	47.48% <0.00%> (ø)
...st/Microsoft.Data.Analysis.Tests/DataFrameTests.cs	99.24% <0.00%> (+<0.01%)	⬆️
...soft.ML.Data/DataLoadSave/Text/TextLoaderCursor.cs	89.77% <0.00%> (+0.15%)	⬆️