AutoML Featurizer handling of label column #6400

Problem

Currently when using the Featurizer with ColumnInformation from ColumnInferenceResults there is no easy way to exclude the label column from featurization or automatically featurize it as well. Therefore you either need to pass in an IDataView with the dropped columns or you need to individually provide the column type parameters (i.e. numericColumns).

var pipeline =
    ctx.Auto().Featurizer(
        data: dataView,
        numericColumns: columnInferenceResults.ColumnInformation.NumericColumnNames.ToArray(),
        outputColumnName: "Features",
        excludeColumns: new[] { "Label" })
        .Append(ctx.Auto().MultiClassification());

Proposal

1. Automatically featurize label column (this is often the case for classification scenarios where values need to be converted to KeyType).

2. Create an overload of Featurizer with the following signature:

Featurizer(IDataView data, ColumnInformation columnInformation, string[] excludeColumns, string outputColumnName)