@Override
  protected void handleInit(Context context) {
    transformFn.init(context);
  }
A simple operator that accepts a 1:n transform function and applies it to each incoming message.

@param <M>  the type of input message
@param <RM>  the type of result
samza-core/src/main/java/org/apache/samza/operators/impl/FlatmapOperatorImpl.java