static int concatDimensionSize(CellVectorMapMap data) {
            Set<Integer> sizes = new HashSet<>();
            data.map.forEach((m, cvmap) ->
                                     cvmap.map.forEach((e, vector) ->
                                                               sizes.add(vector.values.size())));
            if (sizes.isEmpty()) {
                return 1;
            }
            if (sizes.size() == 1) {
                return sizes.iterator().next();
            }
            throw new IllegalArgumentException("inconsistent size of concat dimension, had "+sizes.size()+" different values");
        }
Maps the content in the given list to the given array, using the given index map.

@return true if the mapping was successful, false if one of the destination positions was
        occupied by a different value
vespajlib/src/main/java/com/yahoo/tensor/functions/Concat.java