A Learner that encapsulates a Graph to be used in
mlr3 resampling and benchmarks.
The Graph must return a single Prediction on its $predict()
call. The result of the $train() call is discarded, only the
internal state changes during training are used.
Note the predict_type of a GraphLearner does currently not track the predict_type
of any Learner encapsulated within the Graph. Therefore, when requesting
e.g. "prob" predictions, the predict_type of both the encapsulated Learner
and the wrapping GraphLearner need to be set to "prob".
R6Class object inheriting from mlr3::Learner.
Other Learners: 
mlr_learners_avg