Refactor LLMEngine demo script for clarity and modularity (#1413)

Co-authored-by: Zhuohan Li <zhuohan123@gmail.com>

examples/llm_engine_example.py

 import argparse
+from typing import List, Tuple
 
-from vllm import EngineArgs, LLMEngine, SamplingParams
+from vllm import EngineArgs, LLMEngine, SamplingParams, RequestOutput
 
 
-def main(args: argparse.Namespace):
-    # Parse the CLI argument and initialize the engine.
-    engine_args = EngineArgs.from_cli_args(args)
-    engine = LLMEngine.from_engine_args(engine_args)
-
-    # Test the following prompts.
-    test_prompts = [
+def create_test_prompts() -> List[Tuple[str, SamplingParams]]:
+    """Create a list of test prompts with their sampling parameters."""
+    return [
         ("A robot may not injure a human being",
          SamplingParams(temperature=0.0, logprobs=1, prompt_logprobs=1)),
         ("To be or not to be,",
@@ -25,22 +22,36 @@ def main(args: argparse.Namespace):
                         temperature=0.0)),
     ]
 
-    # Run the engine by calling `engine.step()` manually.
+
+def process_requests(engine: LLMEngine,
+                     test_prompts: List[Tuple[str, SamplingParams]]):
+    """Continuously process a list of prompts and handle the outputs."""
     request_id = 0
-    while True:
-        # To test continuous batching, we add one request at each step.
+
+    while test_prompts or engine.has_unfinished_requests():
         if test_prompts:
             prompt, sampling_params = test_prompts.pop(0)
             engine.add_request(str(request_id), prompt, sampling_params)
             request_id += 1
 
-        request_outputs = engine.step()
+        request_outputs: List[RequestOutput] = engine.step()
+
         for request_output in request_outputs:
             if request_output.finished:
                 print(request_output)
 
-        if not (engine.has_unfinished_requests() or test_prompts):
-            break
+
+def initialize_engine(args: argparse.Namespace) -> LLMEngine:
+    """Initialize the LLMEngine from the command line arguments."""
+    engine_args = EngineArgs.from_cli_args(args)
+    return LLMEngine.from_engine_args(engine_args)
+
+
+def main(args: argparse.Namespace):
+    """Main function that sets up and runs the prompt processing."""
+    engine = initialize_engine(args)
+    test_prompts = create_test_prompts()
+    process_requests(engine, test_prompts)
 
 
 if __name__ == '__main__':