fix: added multiclass init score support

[imatiach-msft]

added multiclass init score support
fixes issue #802

[imatiach-msft]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[codecov]

Codecov Report

Merging #805 into master will increase coverage by 41.29%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           master     #805       +/-   ##
===========================================
+ Coverage   39.19%   80.49%   +41.29%     
===========================================
  Files         241      241               
  Lines        9712     9724       +12     
  Branches      518      530       +12     
===========================================
+ Hits         3807     7827     +4020     
+ Misses       5905     1897     -4008

Impacted Files	Coverage Δ
...a/com/microsoft/ml/spark/lightgbm/TrainUtils.scala	90.9% <100%> (+90.9%)	⬆️
.../microsoft/ml/spark/lightgbm/LightGBMDataset.scala	96.15% <100%> (+96.15%)	⬆️
...com/microsoft/ml/spark/lightgbm/LightGBMBase.scala	84.31% <100%> (+84.31%)	⬆️
src/main/python/mmlspark/io/image/ImageUtils.py	0% <0%> (ø)	⬆️
src/main/python/mmlspark/recommendation/SAR.py	73.33% <0%> (ø)	⬆️
...rc/main/python/mmlspark/lightgbm/LightGBMRanker.py	61.11% <0%> (ø)	⬆️
...python/mmlspark/core/schema/TypeConversionUtils.py	55% <0%> (ø)	⬆️
src/main/python/mmlspark/io/IOImplicits.py	65.27% <0%> (ø)	⬆️
src/main/python/mmlspark/lightgbm/__init__.py	100% <0%> (ø)	⬆️
src/main/python/mmlspark/core/__init__.py	100% <0%> (ø)	⬆️
... and 167 more

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[imatiach-msft]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).