optimize: eliminate duplicate split_enc_dec_inputs calls (#25573)

Signed-off-by: nicole-lihui <nicole.li@daocloud.io>

vllm/v1/engine/processor.py

@@ -388,9 +388,9 @@ class Processor:
 
         eos_token_id = self.input_preprocessor.get_eos_token_id()
 
-        self._validate_model_inputs(processed_inputs)
-
         encoder_inputs, decoder_inputs = split_enc_dec_inputs(processed_inputs)
+        self._validate_model_inputs(encoder_inputs, decoder_inputs)
+
         # Mypy does not always properly infer the types of some elements of
         # discriminated unions of TypedDicts, because of how it handles
         # inheritance of TypedDict. If we explicitly extract the items we want
@@ -458,9 +458,8 @@ class Processor:
             trace_headers=trace_headers,
         )
 
-    def _validate_model_inputs(self, inputs: ProcessorInputs):
-        encoder_inputs, decoder_inputs = split_enc_dec_inputs(inputs)
-
+    def _validate_model_inputs(self, encoder_inputs: Optional[SingletonInputs],
+                               decoder_inputs: SingletonInputs):
         if encoder_inputs is not None:
             self._validate_model_input(encoder_inputs, prompt_type="encoder")