[fix]修复llm_engine.py 越界报错

vllm/engine/llm_engine.py

@@ -989,6 +989,7 @@ class LLMEngine:
                 output = [outputs_by_sequence_group[0][i]]
 
             # tree style speculative decoding may generate empty output in first step
+            if self.speculative_config and self.speculative_config.tree_style_spec_decoding:
                 if outputs and isinstance(output[0], CompletionSequenceGroupOutput):
                     samples = [o.samples[0] for o in output]
                     valid_samples = [