Provide an example of LightgbmRanker?

[sharpwood]

@codemzs

[Zruty0]

LightGBM should appear in the same way as other learners we have (SDCA, FastTree etc.)
Write xmldoc for it in the source code.

[justinormont]

When making an example... ranking has a few differences vs. Regression/Classification tasks:

• Sorted/grouped dataset (all examples from the same group need to be next to each other)
• GroupId is required
• Label is one of { 0, 1, 2, 3, 4 }

There is a benchmark which includes LightGBMRanking:

machinelearning/test/Microsoft.ML.Benchmarks/Numeric/Ranking.cs

Lines 51 to 67 in 17ee205

	[Benchmark]
	public void TrainTest_Ranking_MSLRWeb10K_RawNumericFeatures_LightGBMRanking()
	{
	string cmd = @"TrainTest test=" + _mslrWeb10k_Validate +
	" eval=RankingEvaluator{t=10}" +
	" data=" + _mslrWeb10k_Train +
	" loader=TextLoader{col=Label:R4:0 col=GroupId:TX:1 col=Features:R4:2-138}" +
	" xf=HashTransform{col=GroupId}" +
	" xf=NAHandleTransform{col=Features}" +
	" tr=LightGBMRanking{}";
	using (var environment = EnvironmentFactory.CreateRankingEnvironment<RankerEvaluator, TextLoader, HashTransformer, LightGbmMulticlassTrainer>())
	{
	Maml.MainCore(environment, cmd, alwaysPrintStacktrace: false);
	}
	}
	}

It is written in the command-line language instead of C#. It would be nice to have a C# API ranking example.

[sfilipi]

A test for LightGBM ranker is here: https://github.com/dotnet/machinelearning/blob/master/test/Microsoft.ML.Tests/TrainerEstimators/TreeEstimators.cs#L116

Should convert it to an example.