title: pull xcom in BigQueryOperator
body: <p>I'm trying to run a BigQueryOperator with some dynamic parameter based on a previous task using xcom ( I managed to push it using BashOperator with xcom_push=True)</p>
  
  <p>I thought using the following would do the trick</p>
  
  <pre><code>def get_next_run_date(**context):
      last_date = context['task_instance'].xcom_pull(task_ids=['get_autoplay_last_run_date'])[0].rstrip()
      last_date = datetime.strptime(last_date, "%Y%m%d").date()
      return last_date + timedelta(days=1)
  
  t3 = BigQueryOperator(
      task_id='autoplay_calc',
      bql='autoplay_calc.sql',
      params={
              "env" : deployment
              ,"region" : region
              ,"partition_start_date" : get_next_run_date()
              },
      bigquery_conn_id='gcp_conn',
      use_legacy_sql=False,
      write_disposition='WRITE_APPEND',
      allow_large_results=True,
      #provide_context=True,
      destination_dataset_table=reporting_project + '.pa_reporting_public_batch.autoplay_calc',
      dag=dag
      )` 
  </code></pre>
  
  <p>but using the above provide me with a Broken Dag Error with 'task_instance' error.</p>