rns the mean and std predictions conditional on group_by

    Params
    ------
    preds
        predictions as returned by Vset.predict or Vset.predict_proba
    group_by: list (optional), default None
        list of groups to compute statistics upon

    TODO: Wrap output dicts in dict wrapper::XXX
          Wrap subkeys in Subkey
          Fix default group_by when averaging over all predictions
    &#34;&#34;&#34;
    preds_df = dict_to_df(preds)
    if group_by is None:
        # just average over al