KVOutputAggregator: Fix handling of empty output

[orozery]

This PR fixes KVOutputAggregator to correctly handle the case where ModelRunnerOutput.kv_connector_output is None.

Example trace of bug when loading a single request from KV connector, which yields EmptyModelRunnerOutput for a couple of engine steps:

(EngineCore_0 pid=75768) ERROR 08-14 08:46:12 [core.py:701] EngineCore encountered a fatal error.
(EngineCore_0 pid=75768) ERROR 08-14 08:46:12 [core.py:701] Traceback (most recent call last):
(EngineCore_0 pid=75768) ERROR 08-14 08:46:12 [core.py:701]   File "/workspace/vllm/vllm/v1/engine/core.py", line 692, in run_engine_core
(EngineCore_0 pid=75768) ERROR 08-14 08:46:12 [core.py:701]     engine_core.run_busy_loop()
(EngineCore_0 pid=75768) ERROR 08-14 08:46:12 [core.py:701]   File "/workspace/vllm/vllm/v1/engine/core.py", line 719, in run_busy_loop
(EngineCore_0 pid=75768) ERROR 08-14 08:46:12 [core.py:701]     self._process_engine_step()
(EngineCore_0 pid=75768) ERROR 08-14 08:46:12 [core.py:701]   File "/workspace/vllm/vllm/v1/engine/core.py", line 744, in _process_engine_step
(EngineCore_0 pid=75768) ERROR 08-14 08:46:12 [core.py:701]     outputs, model_executed = self.step_fn()
(EngineCore_0 pid=75768) ERROR 08-14 08:46:12 [core.py:701]                               ^^^^^^^^^^^^^^
(EngineCore_0 pid=75768) ERROR 08-14 08:46:12 [core.py:701]   File "/workspace/vllm/vllm/v1/engine/core.py", line 288, in step
(EngineCore_0 pid=75768) ERROR 08-14 08:46:12 [core.py:701]     model_output = self.execute_model_with_error_logging(
(EngineCore_0 pid=75768) ERROR 08-14 08:46:12 [core.py:701]                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
(EngineCore_0 pid=75768) ERROR 08-14 08:46:12 [core.py:701]   File "/workspace/vllm/vllm/v1/engine/core.py", line 274, in execute_model_with_error_logging
(EngineCore_0 pid=75768) ERROR 08-14 08:46:12 [core.py:701]     raise err
(EngineCore_0 pid=75768) ERROR 08-14 08:46:12 [core.py:701]   File "/workspace/vllm/vllm/v1/engine/core.py", line 265, in execute_model_with_error_logging
(EngineCore_0 pid=75768) ERROR 08-14 08:46:12 [core.py:701]     return model_fn(scheduler_output)
(EngineCore_0 pid=75768) ERROR 08-14 08:46:12 [core.py:701]            ^^^^^^^^^^^^^^^^^^^^^^^^^^
(EngineCore_0 pid=75768) ERROR 08-14 08:46:12 [core.py:701]   File "/workspace/vllm/vllm/v1/executor/multiproc_executor.py", line 192, in execute_model
(EngineCore_0 pid=75768) ERROR 08-14 08:46:12 [core.py:701]     return self.kv_output_aggregator.aggregate(outputs, self.output_rank)
(EngineCore_0 pid=75768) ERROR 08-14 08:46:12 [core.py:701]            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
(EngineCore_0 pid=75768) ERROR 08-14 08:46:12 [core.py:701]   File "/workspace/vllm/vllm/distributed/kv_transfer/kv_connector/utils.py", line 146, in aggregate
(EngineCore_0 pid=75768) ERROR 08-14 08:46:12 [core.py:701]     update_finished_set(output.finished_sending,
(EngineCore_0 pid=75768) ERROR 08-14 08:46:12 [core.py:701]                         ^^^^^^^^^^^^^^^^^^^^^^^
(EngineCore_0 pid=75768) ERROR 08-14 08:46:12 [core.py:701] AttributeError: 'NoneType' object has no attribute 'finished_sending'

[gemini-code-assist]

Code Review

This pull request correctly fixes a critical AttributeError in KVOutputAggregator when kv_connector_output is None. The added check is a good solution. However, I found another critical issue in the same aggregate method: it will crash with an IndexError if the input outputs list is empty. This can occur if async_aggregate receives an empty list of futures. Since the goal is to handle empty outputs robustly, I strongly recommend adding a check for an empty outputs list at the start of the aggregate method. As this is outside the changed lines, I'm mentioning it in this general feedback.

[github-actions]

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[orozery]

@njhill

[orozery]

Already fixed in #22663