[core] simplify seq group code (#9569)

Co-authored-by: Zhuohan Li <zhuohan123@gmail.com>

vllm/engine/llm_engine.py

@@ -647,10 +647,24 @@ class LLMEngine:
         prompt_adapter_request: Optional[PromptAdapterRequest],
         trace_headers: Optional[Mapping[str, str]] = None,
         priority: int = 0,
-    ) -> SequenceGroup:
+    ) -> Optional[SequenceGroup]:
         """Add a processed request to the engine's request pool.
         return the created sequence group.
         """
+        if isinstance(params, SamplingParams) and params.n > 1:
+            ParallelSampleSequenceGroup.add_request(
+                request_id,
+                self,
+                params,
+                processed_inputs=processed_inputs,
+                arrival_time=arrival_time,
+                lora_request=lora_request,
+                trace_headers=trace_headers,
+                prompt_adapter_request=prompt_adapter_request,
+                priority=priority,
+            )
+            return None
+
         self._validate_model_inputs(processed_inputs)
         # Create the sequences.
         block_size = self.cache_config.block_size
@@ -721,7 +735,7 @@ class LLMEngine:
         trace_headers: Optional[Mapping[str, str]] = None,
         prompt_adapter_request: Optional[PromptAdapterRequest] = None,
         priority: int = 0,
-    ) -> Optional[SequenceGroup]:
+    ) -> None:
         ...
 
     @overload
@@ -735,7 +749,7 @@ class LLMEngine:
         trace_headers: Optional[Mapping[str, str]] = None,
         prompt_adapter_request: Optional[PromptAdapterRequest] = None,
         priority: int = 0,
-    ) -> Optional[SequenceGroup]:
+    ) -> None:
         ...
 
     @deprecate_kwargs(
@@ -754,7 +768,7 @@ class LLMEngine:
             priority: int = 0,
             *,
             inputs: Optional[PromptType] = None,  # DEPRECATED
-    ) -> Optional[SequenceGroup]:
+    ) -> None:
         """Add a request to the engine's request pool.
 
         The request is added to the request pool and will be processed by the
@@ -798,22 +812,6 @@ class LLMEngine:
             >>> # continue the request processing
             >>> ...
         """
-
-        if isinstance(params, SamplingParams) and params.n > 1:
-            ParallelSampleSequenceGroup.add_request(
-                request_id,
-                self,
-                params,
-                prompt=prompt,
-                arrival_time=arrival_time,
-                lora_request=lora_request,
-                trace_headers=trace_headers,
-                prompt_adapter_request=prompt_adapter_request,
-                priority=priority,
-                inputs=inputs,
-            )
-            return None
-
         if inputs is not None:
             prompt = inputs
         assert prompt is not None and params is not None
@@ -844,7 +842,7 @@ class LLMEngine:
         processed_inputs["mm_processor_kwargs"] = preprocessed_inputs.get(
             "mm_processor_kwargs")
 
-        return self._add_processed_request(
+        self._add_processed_request(
             request_id=request_id,
             processed_inputs=processed_inputs,
             params=params,