[rollout] feat: use dummy load_format when init AsyncServer (volcengine#3184)

### What does this PR do?

- Loading weights in AsyncServer is duplicated and is time-consuming for
large models
- Use dummy weights instead as the actual weights will be transferred by
the trainer

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Author: vermouth1992

verl/workers/rollout/vllm_rollout/vllm_async_server.py

@@ -276,7 +276,7 @@ async def init_engine(self):
             skip_tokenizer_init=False,
             max_model_len=self.max_model_len,
             max_num_seqs=config.max_num_seqs,
-            load_format="auto",
+            load_format=config.load_format,
             disable_log_stats=config.disable_log_stats,
             max_num_batched_tokens=max_num_batched_tokens,
             enable_chunked_prefill=config.enable_chunked_prefill,