[Bugfix][Misc]: fix graph capture for decoder (#9549)

Author: yudian0504

vllm/worker/model_runner.py

@@ -828,7 +828,7 @@ def build(self) -> ModelInputForGPU:
 
         cuda_graph_pad_size = self._get_cuda_graph_pad_size(
             num_seqs=len(seq_lens),
-            max_decode_seq_len=max_encoder_seq_len,
+            max_decode_seq_len=max_decode_seq_len,
             max_encoder_seq_len=max_encoder_seq_len)
 
         batch_size = len(input_tokens)