[sglang] fix: fix missing engine_kwargs (#2823)

### What does this PR do?

- fix missing engine_kwargs that causes CI on main to fail

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---------

Co-authored-by: gaoziyuan <[email protected]>

Author: vermouth1992

verl/workers/rollout/sglang_rollout/sglang_rollout.py

@@ -438,7 +438,8 @@ def _init_inference_engine(self, trust_remote_code, actor_module, port):
         tp_size_per_node = self._tp_size // nnodes
         node_rank = self._tp_rank // tp_size_per_node
         first_rank_in_node = self._tp_rank % tp_size_per_node == 0
-        attention_backend = self.config.engine_kwargs.sglang.attention_backend
+        engine_kwargs = self.config.get("engine_kwargs", {}).get("sglang", {})
+        attention_backend = engine_kwargs.get("attention_backend", None)
 
         if first_rank_in_node:
             rank = dist.get_rank()