[Misc][V0 Deprecation] Add __main__ guard to all offline examples (#1837)

### What this PR does / why we need it?
Add `__main__` guard to all offline examples.

- vLLM version: v0.9.2
- vLLM main:
76b494444f

---------

Signed-off-by: shen-shanshan <467638484@qq.com>

examples/offline_inference_sleep_mode_npu.py

@@ -25,7 +25,7 @@ from vllm.utils import GiB_bytes
 os.environ["VLLM_USE_MODELSCOPE"] = "True"
 os.environ["VLLM_WORKER_MULTIPROC_METHOD"] = "spawn"
 
-if __name__ == "__main__":
+def main():
     prompt = "How are you?"
 
     free, total = torch.npu.mem_get_info()
@@ -51,3 +51,7 @@ if __name__ == "__main__":
     output2 = llm.generate(prompt, sampling_params)
     # cmp output
     assert output[0].outputs[0].text == output2[0].outputs[0].text
+
+
+if __name__ == "__main__":
+    main()