code: def test_multiple_callbacks(ray_start_1_cpu):
      """
      Test that multiple callbacks are supported.
      """
  
      class PostsubmitCallback(RayDaskCallback):
          def __init__(self, postsubmit_actor):
              self.postsubmit_actor = postsubmit_actor
  
          def _ray_postsubmit(self, task, key, deps, object_ref):
              self.postsubmit_actor.postsubmit.remote(
                  task, key, deps, object_ref
              )
  
      @ray.remote
      class PostsubmitActor:
          def __init__(self):
              self.postsubmit_counter = 0
  
          def postsubmit(self, task, key, deps, object_ref):
              self.postsubmit_counter += 1
  
          def get_postsubmit_counter(self):
              return self.postsubmit_counter
  
      postsubmit_actor = PostsubmitActor.remote()
      cb1 = PostsubmitCallback(postsubmit_actor)
      cb2 = PostsubmitCallback(postsubmit_actor)
      cb3 = PostsubmitCallback(postsubmit_actor)
      with cb1, cb2, cb3:
          z = add(2, 3)
          result = z.compute(scheduler=ray_dask_get)
  
      assert (
          ray.get(postsubmit_actor.get_postsubmit_counter.remote()) == 3
      )
      assert result == 5
  
file_path: ray/python/ray/util/dask/tests/test_dask_callback.py
imports: import dask
  import pytest
  import ray
  from ray.util.dask import ray_dask_get, RayDaskCallback
label_desc: Use image node_id 9 for calling a global function with example usage: test_multiple_callbacks(ray_start_1_cpu) and returns: self