public IndexedTensor.DirectIndexBuilder denseSubspaceBuilder(TensorAddress sparseAddress) {
            double[] values = new double[(int)denseSubspaceSize()];
            denseSubspaceMap.put(sparseAddress, values);
            return new DenseSubspaceBuilder(denseSubtype, values);
        }
For each sparse partial address, hold a dense subspace
vespajlib/src/main/java/com/yahoo/tensor/MixedTensor.java