Init _bestMetricValue for Loss metric

[MiroslavKabat]

_bestMetricValue must be initialized with Single.MaxValue for Loss metric because is decreasing over training.

Fixes #5938

[michaelgsharp]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[michaelgsharp]

LGTM. I restarted the CI process, so I'll merge it in when its done.

[codecov]

Codecov Report

Merging #5939 (3d6d215) into main (3bf8cba) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #5939      +/-   ##
==========================================
+ Coverage   68.19%   68.23%   +0.04%     
==========================================
  Files        1142     1142              
  Lines      242367   242802     +435     
  Branches    25355    25385      +30     
==========================================
+ Hits       165282   165682     +400     
- Misses      70406    70425      +19     
- Partials     6679     6695      +16     

Flag	Coverage Δ
Debug	68.23% <100.00%> (+0.04%)	⬆️
production	62.89% <100.00%> (+0.01%)	⬆️
test	88.61% <ø> (+0.04%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
.../Microsoft.ML.Vision/ImageClassificationTrainer.cs	92.25% <100.00%> (+0.61%)	⬆️
...ansforms/PermutationFeatureImportanceExtensions.cs	92.30% <0.00%> (-3.57%)	⬇️
...rosoft.ML.AutoML/ColumnInference/TextFileSample.cs	59.60% <0.00%> (-2.65%)	⬇️
src/Microsoft.ML.Sweeper/AsyncSweeper.cs	71.42% <0.00%> (-1.37%)	⬇️
...soft.ML.Transforms/Text/WordEmbeddingsExtractor.cs	85.74% <0.00%> (-1.14%)	⬇️
src/Microsoft.Data.Analysis/Strings.Designer.cs	43.26% <0.00%> (-0.95%)	⬇️
.../Microsoft.ML.Data/Prediction/CalibratorCatalog.cs	93.33% <0.00%> (-0.49%)	⬇️
...rosoft.Data.Analysis/ArrowStringDataFrameColumn.cs	63.54% <0.00%> (-0.47%)	⬇️
src/Microsoft.ML.OnnxTransformer/OnnxTypeParser.cs	64.36% <0.00%> (-0.35%)	⬇️
...StandardTrainers/Standard/LinearModelParameters.cs	65.82% <0.00%> (-0.26%)	⬇️
... and 20 more