Pipeline Parallel: Guard for KeyErrors at request abort (#6587)

Signed-off-by: Travis Johnson <tsjohnso@us.ibm.com>

vllm/engine/async_llm_engine.py

@@ -131,7 +131,10 @@ class RequestTracker:
         """Process a request output from the engine."""
         request_id = request_output.request_id
 
-        self._request_streams[request_id].put(request_output)
+        # Guard against a KeyError which can occur if the request was aborted
+        # while the output was generated
+        if (stream := self._request_streams.get(request_id)) is not None:
+            stream.put(request_output)
         if request_output.finished:
             if verbose:
                 logger.info("Finished request %s.", request_id)