title: DoFn process return type
body: <p>In Beam 2.3.0, the <code>DoFn</code> process API needed to return a generator through <code>yield</code> while in Beam 2.4.0, returning an object like dictionary from the process API works fine. </p>
  
  <p>Is there a change in programming model from 2.3.0 to 2.4.0 ? For large scale Google Cloud Dataflow processing, is <code>yield</code> (generator) preferred over returning objects (list, dict) from the DoFn process API ?</p>