Adds PriorTrainer Onnx conversion

[Lynx1820]

1. Adding Onnx support for PriorTrainer
   To follow the onnx construction pattern of other binary classifiers, this trainer uses the probability to predict labels, instead of score

[harishsk]

🕐

[codecov]

Codecov Report

Merging #4515 into master will decrease coverage by <.01%.
The diff coverage is 97.77%.

@@            Coverage Diff             @@
##           master    #4515      +/-   ##
==========================================
- Coverage   75.12%   75.12%   -0.01%     
==========================================
  Files         909      909              
  Lines      160267   160305      +38     
  Branches    17257    17258       +1     
==========================================
+ Hits       120406   120427      +21     
- Misses      35049    35060      +11     
- Partials     4812     4818       +6

Flag	Coverage Δ
#Debug	75.12% <97.77%> (-0.01%)	⬇️
#production	70.52% <97.61%> (-0.01%)	⬇️
#test	90.29% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
...Microsoft.ML.Data/Scorers/PredictionTransformer.cs	91.07% <100%> (+0.18%)	⬆️
test/Microsoft.ML.Tests/OnnxConversionTest.cs	95.54% <100%> (ø)	⬆️
...StandardTrainers/Standard/Simple/SimpleTrainers.cs	84.9% <100%> (+2.81%)	⬆️
....ML.Data/Scorers/SchemaBindablePredictorWrapper.cs	74.52% <83.33%> (ø)	⬆️
...c/Microsoft.ML.FastTree/Utils/ThreadTaskManager.cs	79.48% <0%> (-20.52%)	⬇️
....ML.AutoML/PipelineSuggesters/PipelineSuggester.cs	83.19% <0%> (-3.37%)	⬇️
...soft.ML.Transforms/Text/WordEmbeddingsExtractor.cs	86.39% <0%> (-1.14%)	⬇️
src/Microsoft.ML.AutoML/Sweepers/Parameters.cs	84.32% <0%> (-0.85%)	⬇️
...soft.ML.Data/DataLoadSave/Text/TextLoaderCursor.cs	84.7% <0%> (-0.41%)	⬇️
src/Microsoft.ML.Transforms/Text/LdaTransform.cs	84.6% <0%> (+0.15%)	⬆️
... and 1 more

[harishsk]