Make num_top_classes parameter's default value equal to 2

[przemekwitek]

Users are often interested in the probabilities of each class being predicted.
This PR sets the default of num_top_classes parameter (which controls how many classes with probabilities are returned) to 2.

Relates to #46735

[elasticmachine]

Pinging @elastic/ml-core (:ml)

[benwtrent]

This does not really make sense for multi-class as returning the top two will seem arbitrary in that situation.

Maybe just the probability of the one we return (i.e. numTopClasses = 1)?

One could see a parallel between this setting and the computeFeatureInfluence and featureInfluenceThreshold settings for outlier_detection.

It almost seems like we need a "close to" parameter that returns all the top classes that are above a certain probability... @tveasey what say you?

[przemekwitek]

This does not really make sense for multi-class as returning the top two will seem arbitrary in that situation.

I agree. Please remember though that in 7.5 we don't go multiclass and for binary "2" is a sensible default.

Maybe just the probability of the one we return (i.e. numTopClasses = 1)?

Let's revisit it in 7.6 when we go multiclass.

One could see a parallel between this setting and the computeFeatureInfluence and featureInfluenceThreshold settings for outlier_detection.

It almost seems like we need a "close to" parameter that returns all the top classes that are above a certain probability... @tveasey what say you?

[benwtrent]

I am OK with this, we just need to document that the default value is 2 somewhere and that supplying a 0 is needed to prevent them from being returned.

I do think that a parameter like top_classes_threshold for the user to provide a probability threshold and return the top classes that are above that threshold would be a great addition. I will open an separate issue for that :)

[przemekwitek]

run elasticsearch-ci/packaging-sample
run elasticsearch-ci/packaging-sample-matrix

[przemekwitek]

run elasticsearch-ci/packaging-sample
run elasticsearch-ci/packaging-sample-matrix

[przemekwitek]

run elasticsearch-ci/packaging-sample-matrix