[core][cgraph] Fix flaky test_dag_exception_chained

[ruisearch42]

Why are these changes needed?

test_dag_exception_chained has the following exception, which causes the next test to fail:

[2025-02-05T01:46:17Z] Exception ignored in: <function CompiledDAG.__del__ at 0x7fc7e6831a80>
--
  | [2025-02-05T01:46:17Z] Traceback (most recent call last):
  | [2025-02-05T01:46:17Z]   File "/rayci/python/ray/dag/compiled_dag_node.py", line 2957, in __del__
  | [2025-02-05T01:46:17Z]     self.teardown()
  | [2025-02-05T01:46:17Z]   File "/rayci/python/ray/dag/compiled_dag_node.py", line 2949, in teardown
  | [2025-02-05T01:46:17Z]     monitor.teardown(kill_actors=kill_actors)
  | [2025-02-05T01:46:17Z]   File "/rayci/python/ray/dag/compiled_dag_node.py", line 1998, in teardown
  | [2025-02-05T01:46:17Z]     outer._dag_submitter.close()
  | [2025-02-05T01:46:17Z]     ^^^^^^^^^^^^^^^^^^^^
  | [2025-02-05T01:46:17Z] ReferenceError: weakly-referenced object no longer exists

Related issue number

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• Testing Strategy
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[dayshah]

do you know when this error started happening?

[dayshah]

shouldn't this protect it from happening if we already destructed?

[ruisearch42]

I think teardown() is only entered once, but CompiledDAG attributes may already be partially destructed when __del__() is called

[dayshah]

oh ok got it