Fixes onnx exports for regression trainers

[kere-nel]

This PR fixes the issues with exporting the following regression trainers to onnx:

• LbfgsPoissonRegression
• FastTreeTweedie
• FastForest

And adds onnx conversion tests for the following regression trainers:

• FastTree
• OnlineGradientDescent
• Ols
• LightGbm
• Sdca

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@1fa6cb5). Click here to learn what that means.
The diff coverage is 90.47%.

@@            Coverage Diff            @@
##             master    #4448   +/-   ##
=========================================
  Coverage          ?   74.84%           
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  Files             ?      908           
  Lines             ?   159980           
  Branches          ?    17236           
=========================================
  Hits              ?   119744           
  Misses            ?    35404           
  Partials          ?     4832

Flag	Coverage Δ
#Debug	74.84% <90.47%> (?)
#production	70.22% <100%> (?)
#test	90.08% <82.97%> (?)

Impacted Files	Coverage Δ
src/Microsoft.ML.FastTree/FastTree.cs	80.64% <100%> (ø)
...StandardTrainers/Standard/LinearModelParameters.cs	61.06% <100%> (ø)
src/Microsoft.ML.FastTree/FastTreeTweedie.cs	58.02% <100%> (ø)
...rc/Microsoft.ML.FastTree/RandomForestRegression.cs	70.43% <100%> (ø)
test/Microsoft.ML.Tests/OnnxConversionTest.cs	94.49% <82.97%> (ø)