Evaluation result classes shouldn't be nested under the Evaluator classes

[eerhardt]

When using our evaluation API:

BinaryClassificationContext binClassificationCtx = new BinaryClassificationContext(env);
BinaryClassifierEvaluator.CalibratedResult metrics = binClassificationCtx.Evaluate(predictions, "Label");

It is quite odd for the metrics object to be a nested class in the BinaryClassifierEvaluator. I didn't even use the BinaryClassifierEvaluator class at all, why am I dealing with a nested class inside of it?

I think these result classes should be moved to no longer be nested classes. Instead we should just have a BinaryClassifierEvaluationResult class.

/cc @Zruty0 @TomFinley @KrzysztofCwalina @terrajobst

[Zruty0]

I'm OK with the suggestion, although I would probably call it BinaryClassificationMetrics instead.

[eerhardt]

Good point. That's what we called it in the Legacy LearningPipeline API.

machinelearning/src/Microsoft.ML.Legacy/Models/BinaryClassificationMetrics.cs

Lines 14 to 17 in 65c3c7c

	/// <summary>
	/// This class contains the overall metrics computed by binary classification evaluators.
	/// </summary>
	public sealed class BinaryClassificationMetrics