Split the data according to the indexes in `valid_idx`.

def split_by_idx(self, valid_idx:Collection[int])->'ItemLists':
        "Split the data according to the indexes in `valid_idx`."
        #train_idx = [i for i in range_of(self.items) if i not in valid_idx]
        train_idx = np.setdiff1d(arange_of(self.items), valid_idx)
        return self.split_by_idxs(train_idx, valid_idx)