[Misc] Make handling of SamplingParams clearer in n>1 case (#26032)

Signed-off-by: Nick Hill <nhill@redhat.com>

vllm/v1/engine/async_llm.py

@@ -289,13 +289,19 @@ class AsyncLLM(EngineClient):
             await self._add_request(request, prompt_str, None, 0, queue)
             return queue
 
+        # Get the updated SamplingParams from the request, which
+        # were cloned/updated in processor.process_inputs above.
+        parent_params = request.sampling_params
+        assert parent_params is not None
+
         # Fan out child requests (for n>1).
-        parent_request = ParentRequest(request_id, request.sampling_params)
-        for idx in range(params.n):
-            request_id, params = parent_request.get_child_info(idx)
-            child_request = request if idx == params.n - 1 else copy(request)
+        parent_request = ParentRequest(request_id, parent_params)
+        for idx in range(parent_params.n):
+            request_id, child_params = parent_request.get_child_info(idx)
+            child_request = request if idx == parent_params.n - 1 else copy(
+                request)
             child_request.request_id = request_id
-            child_request.sampling_params = params
+            child_request.sampling_params = child_params
             await self._add_request(child_request, prompt_str, parent_request,
                                     idx, queue)
         return queue