made serviceCollection internal to be accessed by AutoML extension in fairlearn

In fairlearn AutoML, we have to add in a singleton to the serviceCollection called moment, which will be later extracted to calculate the fairness parity.

Author: jordi1215

src/Microsoft.ML.AutoML/AutoMLExperiment/AutoMLExperiment.cs

@@ -197,6 +197,8 @@ public AutoMLExperiment SetEvaluateMetric(RegressionMetric metric, string labelC
             return this;
         }
 
+        internal IServiceCollection ServiceCollection { get => _serviceCollection; }
+
         /// <summary>
         /// Run experiment and return the best trial result synchronizely.
         /// </summary>