feat: add num iteration and start iteration to lightgbm model

[imatiach-msft]

• add num iteration and start iteration to lightgbm model
• set num iteration to be best iteration when early stopping criteria reached
  resolves issue Transform at the best iteration for LightGBMClassifier #1006

[imatiach-msft]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[codecov]

Codecov Report

Merging #1024 (bba7244) into master (2c223f6) will decrease coverage by 0.01%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master    #1024      +/-   ##
==========================================
- Coverage   84.18%   84.17%   -0.02%     
==========================================
  Files         201      201              
  Lines        9306     9338      +32     
  Branches      554      555       +1     
==========================================
+ Hits         7834     7860      +26     
- Misses       1472     1478       +6     

Impacted Files	Coverage Δ
...osoft/ml/spark/lightgbm/LightGBMModelMethods.scala	66.66% <66.66%> (ø)
.../microsoft/ml/spark/lightgbm/LightGBMBooster.scala	87.50% <77.77%> (-0.93%)	⬇️
...a/com/microsoft/ml/spark/lightgbm/TrainUtils.scala	77.27% <83.33%> (-0.03%)	⬇️
...crosoft/ml/spark/lightgbm/LightGBMClassifier.scala	90.58% <100.00%> (+0.22%)	⬆️
...icrosoft/ml/spark/lightgbm/LightGBMConstants.scala	100.00% <100.00%> (ø)
...m/microsoft/ml/spark/lightgbm/LightGBMParams.scala	88.15% <100.00%> (+0.58%)	⬆️
...m/microsoft/ml/spark/lightgbm/LightGBMRanker.scala	60.65% <100.00%> (+1.33%)	⬆️
...icrosoft/ml/spark/lightgbm/LightGBMRegressor.scala	70.00% <100.00%> (+1.25%)	⬆️
...com/microsoft/ml/spark/core/utils/AsyncUtils.scala	65.00% <0.00%> (-5.00%)	⬇️
...a/com/microsoft/ml/spark/io/http/HTTPClients.scala	83.33% <0.00%> (-3.34%)	⬇️
... and 3 more

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[imatiach-msft]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[mhamilton723]

One small nit but other than that LGTM

[imatiach-msft]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).