@Override
    public PrimitiveTensorFunction<NAMETYPE> toPrimitive() {
        return toPrimitive(dimension);
    }
The <i>expand</i> tensor function returns a tensor with a new dimension of
size 1 is added, equivalent to "tensor * tensor(dim_name[1])(1)".

@author lesters
vespajlib/src/main/java/com/yahoo/tensor/functions/Expand.java