[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/disaggregated_prefill/disaggregated_prefill_offline.py

@@ -12,6 +12,9 @@ import os
 import time
 from multiprocessing import Event, Process
 
+os.environ["VLLM_USE_MODELSCOPE"] = "True"
+os.environ["VLLM_WORKER_MULTIPROC_METHOD"] = "spawn"
+
 kv_connector_extra_config = {
     "prefill_device_ips": ["1.2.3.1", "1.2.3.2"],
     "decode_device_ips": ["1.2.3.9", "1.2.3.10"],