title: Run another DAG with TriggerDagRunOperator multiple times
body: <p>i have a DAG (DAG1) where i copy a bunch of files. I would then like to kick off another DAG (DAG2) for each file that was copied. As the number of files copied will vary per DAG1 run, i would like to essentially loop over the files and call DAG2 with the appropriate parameters.</p>
  
  <p>eg:</p>
  
  <pre><code>with DAG( 'DAG1',
          description="copy files over",
          schedule_interval="* * * * *",
          max_active_runs=1
      ) as dag:
  
  
      t_rsync = RsyncOperator( task_id='rsync_data',
          source='/source/',
          target='/destination/' )
  
      t_trigger_preprocessing = TriggerDagRunOperator( task_id='trigger_preprocessing',
          trigger_daq_id='DAG2',
          python_callable=trigger
  
      )
  
      t_rsync &gt;&gt; t_trigger_preprocessing
  </code></pre>
  
  <p>i was hoping to use the python_callable <code>trigger</code> to pull the relevant xcom data from <code>t_rsync</code> and then trigger DAG2; but its not clear to me how to do this.</p>
  
  <p>i would prefer to put the logic of calling DAG2 here to simplify the contents of DAG2 (and also provide stacking schematics with the <code>max_active_runs</code>)</p>