Feature Request: Out-Of-Bag Error should not require storing the model.

[sebffischer]

It is currently necessary to store the $model of a learner in order to access its oob_error.
An example for this is the random forest:

library(mlr3verse)
rr = resample(tsk("iris"), rsmp("classif.ranger"), rsmp("holdout"), store_models = TRUE)
rr$aggregate(msr("oob_error"))

The code below will err if we don't specify store_models = TRUE because the out-of-bag error active binding extracts this information from learner$model, which will fail of the model is not stored.

Instead of making the out-of-bag error active binding access the learner's $model, we can instead support the private function $.extract_oob_error() which will add the out-of-bag error score to the learner's $state, which is even accessible when store_models = FALSE.
This is already implemented for internal validation scores and internal tuned values:

mlr3/R/worker.R

Lines 94 to 100 in 5ffcfee

	if (!is.null(validate)) {
	learner$state$internal_valid_scores = get_private(learner)$.extract_internal_valid_scores()
	learner$state$internal_valid_task_hash = task$internal_valid_task$hash
	}
	if (exists(".extract_internal_tuned_values", get_private(learner))) {
	learner$state$internal_tuned_values = get_private(learner)$.extract_internal_tuned_values()