[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/prompt_embedding_inference.py

@@ -1,8 +1,13 @@
+import os
+
 import torch
 from transformers import (AutoModelForCausalLM, AutoTokenizer,
                           PreTrainedTokenizer)
 from vllm import LLM
 
+os.environ["VLLM_USE_MODELSCOPE"] = "True"
+os.environ["VLLM_WORKER_MULTIPROC_METHOD"] = "spawn"
+
 
 def init_tokenizer_and_llm(model_name: str):
     tokenizer = AutoTokenizer.from_pretrained(model_name)