Add XML doc to the ITrainerEstimator interface (#2847)

Author: jwood803

src/Microsoft.ML.Data/Training/ITrainerEstimator.cs

@@ -4,10 +4,18 @@
 
 namespace Microsoft.ML.Trainers
 {
-    public interface ITrainerEstimator<out TTransformer, out TPredictor> : IEstimator<TTransformer>
-        where TTransformer : ISingleFeaturePredictionTransformer<TPredictor>
-        where TPredictor : class
+    /// <summary>
+    /// Interface for the Trainer Estimator.
+    /// </summary>
+    /// <typeparam name="TTransformer">The type of the transformer returned by the estimator.</typeparam>
+    /// <typeparam name="TModel">The type of the model parameters.</typeparam>
+    public interface ITrainerEstimator<out TTransformer, out TModel> : IEstimator<TTransformer>
+        where TTransformer : ISingleFeaturePredictionTransformer<TModel>
+        where TModel : class
     {
+        /// <summary>
+        /// Gets the <see cref="TrainerInfo"/> information about the trainer.
+        /// </summary>
         TrainerInfo Info { get; }
     }
 }