Added IsotonicCalibrator ONNX Export Support

[mstfbl]

WIP PR to add ONNX export support for the Isotonic Calibrator

Current work remaining is implementing if-then-else conditionals. Intermediate calculations and if-then-else inputs are already implemented.

I will add tests (first written in this commit) for validating IsotonicCalibrator with ONNX and merge with upstream once PR #5289 is merged.

[codecov]

Codecov Report

Merging #5298 into master will increase coverage by 13.94%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #5298       +/-   ##
===========================================
+ Coverage   73.68%   87.62%   +13.94%     
===========================================
  Files        1022      243      -779     
  Lines      190366    44548   -145818     
  Branches    20474     1896    -18578     
===========================================
- Hits       140265    39036   -101229     
+ Misses      44568     5167    -39401     
+ Partials     5533      345     -5188     

Flag	Coverage Δ
#Debug	87.62% <ø> (+13.94%)	⬆️
#production	?
#test	87.62% <ø> (ø)

Impacted Files	Coverage Δ
...rc/Microsoft.ML.Data/Commands/ShowSchemaCommand.cs
src/Microsoft.ML.Core/Data/ModelSaving.cs
src/Microsoft.ML.FastTree/BoostingFastTree.cs
...soft.ML.Data/DataLoadSave/Database/DbExtensions.cs
src/Microsoft.ML.AutoML/API/MLContextExtension.cs
...ft.ML.Data/Evaluators/BinaryClassifierEvaluator.cs
...ft.ML.Data/DataLoadSave/Database/DatabaseSource.cs
...chine/FactorizationMachineInterface.netstandard.cs
src/Microsoft.ML.OnnxConverter/OnnxNodeImpl.cs
...L/TrainerExtensions/RegressionTrainerExtensions.cs
... and 768 more

[codecov]

Codecov Report

Merging #5298 into master will decrease coverage by 0.16%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #5298      +/-   ##
==========================================
- Coverage   73.80%   73.64%   -0.17%     
==========================================
  Files        1022     1022              
  Lines      190617   190466     -151     
  Branches    20488    20475      -13     
==========================================
- Hits       140686   140264     -422     
- Misses      44411    44670     +259     
- Partials     5520     5532      +12     

Flag	Coverage Δ
#Debug	73.64% <0.00%> (-0.17%)	⬇️
#production	69.37% <0.00%> (-0.20%)	⬇️
#test	87.62% <ø> (-0.04%)	⬇️

Impacted Files	Coverage Δ
src/Microsoft.ML.Data/Prediction/Calibrator.cs	74.16% <0.00%> (-7.04%)	⬇️
...c/Microsoft.ML.FastTree/Utils/ThreadTaskManager.cs	79.48% <0.00%> (-20.52%)	⬇️
src/Microsoft.ML.AutoML/Utils/SplitUtil.cs	76.31% <0.00%> (-18.43%)	⬇️
src/Microsoft.ML.AutoML/API/ExperimentBase.cs	66.66% <0.00%> (-9.37%)	⬇️
...osoft.ML.KMeansClustering/KMeansPlusPlusTrainer.cs	83.60% <0.00%> (-7.27%)	⬇️
src/Microsoft.ML.AutoML/API/RankingExperiment.cs	60.60% <0.00%> (-7.14%)	⬇️
src/Microsoft.ML.FastTree/Training/StepSearch.cs	57.42% <0.00%> (-4.96%)	⬇️
src/Microsoft.ML.AutoML/API/AutoCatalog.cs	69.35% <0.00%> (-4.84%)	⬇️
src/Microsoft.ML.Data/Training/TrainerUtils.cs	66.86% <0.00%> (-3.82%)	⬇️
...crosoft.ML.StandardTrainers/Standard/SdcaBinary.cs	85.23% <0.00%> (-3.25%)	⬇️
... and 28 more