ML.NET-242: FastTreeRanking per-iteration loss metrics are empty

When training a FastTreeRanker using the `testFrequency` parameter, it is expected that NDCG is prented every testFrequency iterations. However, instead of NDCG, only empty strings are printed.

The root cause was that the MaxDCG property of the dataset was never calculated, so the NDCG calculation is aborted, leaving an empty string as a result.

This PR fixes the problem by computing the MaxDCG for the dataset when the Tests are defined (so that if the tests are not defined, the MaxDCG will never be calculated).

Closes #242

src/Microsoft.ML.FastTree/FastTreeRanking.cs

@@ -343,6 +343,9 @@ protected override void CustomizedTrainingIteration(RegressionTree tree)
         /// <returns>standard test for the dataset</returns>
         private Test CreateStandardTest(Dataset dataset)
         {
+            if (dataset.MaxDcg.Length == 0)
+                dataset.Skeleton.RecomputeMaxDcg(10);
+
             return new NdcgTest(
                 ConstructScoreTracker(dataset),
                 dataset.Ratings,