Add option to use the same tr/vl split for different replicas

Created by: scarlehoff

Even though we are using a tr/vl split per replica it would be useful to have an option such that the tr/vl split is informed just by the seed in the runcard (and not the combination of the seed in the runcard + the replica).

In practice this will be useful for running n3fit in GPU (where it is necessary to have the data in the same position for all replicas).

I'm wondering, @wilsonmr, whether this option exists already somewhere (since you explored this)?