fix rollout(#4055)

Author: hjh0119

swift/llm/infer/rollout.py

@@ -131,7 +131,7 @@ def get_infer_engine(args: InferArguments, **kwargs):
         })
         infer_backend = kwargs.pop('infer_backend', None) or args.infer_backend
         if infer_backend != 'vllm':
-            infer_backend == 'vllm'
+            infer_backend = 'vllm'
             logger.info('Currently, rollout only supports the vLLM backend. Set vLLM backend')
         use_async_engine = args.use_async_engine
         if use_async_engine: