code: public static TableFactor unity(VariableNumMap vars) {
      return new TableFactor(vars, 
          DenseTensor.constant(vars.getVariableNumsArray(), vars.getVariableSizes(), 1.0));
    }
docs: Gets a {@code TableFactor} over {@code vars} which assigns weight 1 to all
  assignments.
  
  @param vars
  @return