title: What is the best way to see how Dataflow is doing fusion optimization
body: <p>I'm trying to optimize a pipeline that pulls messages from PubSubIO and sends those messages to a 3rd party API. One interesting observation I have is that if I put a <code>GroupBy</code> and a "Degroup" transform after the <code>PubSubIO.read</code>, the throughput of pipeline increased significantly. I added the <code>GroupBy</code> just to prevent fusion optimization, and now I wonder how exactly the transform being merged in a given pipeline.</p>
  
  <p>What is the best way to find out how a pipeline looks like after the fusion?</p>