[V1] Avoid redundant input processing in n>1 case (#14985)

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

vllm/v1/engine/llm_engine.py

@@ -1,6 +1,7 @@
 # SPDX-License-Identifier: Apache-2.0
 
 from collections.abc import Mapping
+from copy import copy
 from typing import Optional, Union
 
 from typing_extensions import TypeVar
@@ -179,25 +180,34 @@ class LLMEngine:
         prompt_adapter_request: Optional[PromptAdapterRequest] = None,
         priority: int = 0,
     ) -> None:
-        # 1) Fan out child requests (for n>1)
-        parent_req = ParentRequest.from_params(request_id, params)
+        # Process raw inputs into the request.
+        request = self.processor.process_inputs(request_id, prompt, params,
+                                                arrival_time, lora_request,
+                                                trace_headers,
+                                                prompt_adapter_request,
+                                                priority)
+
         n = params.n if isinstance(params, SamplingParams) else 1
-        for idx in range(n):
-            if parent_req is not None:
-                request_id, params = parent_req.get_child_info(idx)
 
-            # 2) Process raw inputs into the request.
-            request = self.processor.process_inputs(request_id, prompt, params,
-                                                    arrival_time, lora_request,
-                                                    trace_headers,
-                                                    prompt_adapter_request,
-                                                    priority)
-
-            # 3) Make a new RequestState and queue.
-            self.output_processor.add_request(request, parent_req, idx)
-
-            # 3) Add the request to EngineCore.
+        if n == 1:
+            # Make a new RequestState and queue.
+            self.output_processor.add_request(request, None, 0)
+            # Add the request to EngineCore.
             self.engine_core.add_request(request)
+            return
+
+        # Fan out child requests (for n>1).
+        parent_req = ParentRequest(request_id, params)
+        for idx in range(n):
+            request_id, params = parent_req.get_child_info(idx)
+            child_request = request if idx == n - 1 else copy(request)
+            child_request.request_id = request_id
+            child_request.sampling_params = params
+
+            # Make a new RequestState and queue.
+            self.output_processor.add_request(child_request, parent_req, idx)
+            # Add the request to EngineCore.
+            self.engine_core.add_request(child_request)
 
     def step(self) -> list[RequestOutput]: