R4 label works with some but not all binary classification trainers

[daholste]

This code:

using System;
using Microsoft.ML.Data;

namespace Microsoft.ML.Samples
{
    internal static class Program
    {
        static void Main(string[] args)
        {
            var context = new MLContext();
            var options = new TextLoader.Options()
            {
                Columns = new TextLoader.Column[]
                {
                    new TextLoader.Column("Label", DataKind.Single, 0),
                    new TextLoader.Column("Sentiment", DataKind.String, 1)
                },
                HasHeader = true
            };
            var loader = context.Data.CreateTextLoader(options);
            var data = loader.Read(@"C:\AutoMLDotNet\src\Samples\Data\wikipedia-detox-250-line-data.tsv");
            var estimator = context.Transforms.Text.FeaturizeText("Features", "Sentiment")
                .Append(context.BinaryClassification.Trainers.AveragedPerceptron());
            var transformer = estimator.Fit(data);
            var scoredData = transformer.Transform(data);
            var metrics = context.BinaryClassification.EvaluateNonCalibrated(scoredData);
            Console.WriteLine(metrics.Accuracy);
            Console.WriteLine("Press any key...");
            Console.ReadLine();
        }
    }
}

works great!

If you replace AveragedPerceptron with LogisticRegression, it throws the exception:

 'Schema mismatch for label column: expected Bool, got R4'

This may indicate a bug in label schema validation with some binary learners

[Ivanidzo4ka]

related to #2628

[eerhardt]

I'm not sure this meets "Project 13". I don't see how this would be an API breaking. We can fix this after v1 without a breaking change.

@TomFinley @shauheen