public Descriptor build() {
      return new Descriptor(mode, argCount, argTypes, exprTypes, unscaled);
    }
Builder builds a {@link Descriptor} object. Setter methods are provided to set the {@link Mode}, number
of arguments, argument types and expression types for each argument.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/VectorExpressionDescriptor.java