title: kubeflow pipeline dynamic output list as input parameter
body: <p>I use a ParallelFor over a dynamic list. I want to collect all the outputs from the loop, and pass them to another ContainerOp.<br>
   Something like the following, which obviously does not work, since the <code>outputs</code> list is will be static.</p>
  
  <pre><code>with dsl.ParallelFor(op1.output) as item:
      op2 = dsl.ContainerOp(
        name='op2',
        ...
        file_outputs={
           'outputs': '/outputs.json',
      })
      outputs.append(op2.output)
  
  
  op3 = dsl.ContainerOp(
     name='op3',
     ...
     arguments=['--input': outputs]  # won't work
  )
  </code></pre>