MapValueToKey in ranking sample

[justinormont]

In the ranking sample, we're using a MapValueToKey transform to convert the Label UINT values { 0, 1, 2, 3, 4 } to a Key. This seems incorrect as the MapValueToKey will order the Key by the order it sees them. This will cause the ranker to randomly switch label values, a "good" could used internally as "bad".

MapValueToKey transform in the ranking example:

machinelearning-samples/samples/csharp/getting-started/Ranking_Web/WebRanking/Program.cs

Line 150 in 8697bec

.Append(mlContext.Transforms.Conversion.MapValueToKey(nameof(SearchResultData.Label)))

Work:

1. We should validate that the current sample is working as expected. It may be permuting the labels.
2. Ideally we would expose the ability to directly set the term list for MapValueToKey; I don't see this term list parameter exposed in the estimators API.

[montekristo1946]

Faced a similar problem. In the official example (MulticlassClassification) shuffles the labels.