Init _bestMetricValue for Loss metric (dotnet#5939)

Author: MiroslavKabat

src/Microsoft.ML.Vision/ImageClassificationTrainer.cs

@@ -258,7 +258,10 @@ public EarlyStopping(float minDelta = 0.01f, int patience = 20, EarlyStoppingMet
                 if (metric == EarlyStoppingMetric.Accuracy)
                     CheckIncreasing = true;
                 else if (metric == EarlyStoppingMetric.Loss)
+                {
                     CheckIncreasing = false;
+                    _bestMetricValue = Single.MaxValue;
+                }
             }
 
             /// <summary>