In view to be able to use determinist metrics in `evalp`, the temporal
masks need to feature a dimension specific to each prediction series.
In fact, this was already done internally to `determinist::Evaluator`
to deal with potentially missing predictions. From a determinist point
of view, this is actually beneficial as well since (a) it allows the
user to use different masks for different series and (b) it avoids an
internal copy of masks to go from 2D to 3D.