Takes a lists of Task, a list of Learner and a list of Resampling to
generate a design in an expand.grid() fashion (a.k.a. cross join or Cartesian product).
Resampling strategies may not be instantiated, and will be instantiated per task internally.
expand_grid(tasks, learners, resamplings)(list of Learner | character())
Instead if a Learner object, it is also possible to provide keys to retrieve learners from the mlr_learners dictionary.
:: (list of Resampling | character())
Instead if a Resampling object, it is also possible to provide a key to retrieve a resampling from the mlr_resamplings dictionary.
(data.table()) with the cross product of the input vectors.