code: def notify_flush(self):
      self._flush_event = True
  
file_path: flink/flink-python/pyflink/fn_execution/beam/beam_stream_slow.py
imports: from apache_beam.coders.coder_impl import create_InputStream, create_OutputStream
  from pyflink.fn_execution.stream_slow import InputStream
  from pyflink.fn_execution.utils.operation_utils import PeriodicThread
label_desc: Use image node_id 4 for calling the BeamTimeBasedOutputStream obj's underlying member method code with example usage: obj.notify_flush() without return types