[Bugfix] Validate custom logits processor xargs for online serving (#27560)

Signed-off-by: Isotr0py <mozf@mail2.sysu.edu.cn>

docs/design/logits_processors.md

@@ -254,7 +254,15 @@ The previous sections alluded to the interfaces which vLLM logits processors mus
                 changes to the batch makeup.
             """
             raise NotImplementedError
-            
+
+        @classmethod
+        def validate_params(cls, sampling_params: SamplingParams):
+            """Validate sampling params for this logits processor.
+
+            Raise ValueError for invalid ones.
+            """
+            return None
+
     ```
 
 A vLLM logits processor must subclass `LogitsProcessor` and define (at minimum) the following methods:
@@ -279,6 +287,10 @@ A vLLM logits processor must subclass `LogitsProcessor` and define (at minimum)
     * Use the `BatchUpdate` members to update logits processor internal state
     * **Note:** batch update data structure may be `None`, signaling no change to the batch constituents. In this case, the LogitsProcessor might still want to update its state based on the updated `output_token_ids` lists that it could have retained when they were added.
 
+* `validate_params(cls, sampling_params: SamplingParams)`:
+    * Raise `ValueError` if `SamplingParams` has invalid arguments (especially custom arguments) used by logits processor.
+    * When request is sent to entrypoint, `validate_params()` will validate `SamplingParams` and refuse request with invalid arguments.
+
 ### `BatchUpdate` data structure
 
 The `BatchUpdate` abstraction models the persistent batch as a list of requests, supporting the following operations to change batch state (note that the order in which the operations are mentioned below reflects the order in which they should be processed in `update_state()`):