[Example] Add `async_llm_streaming.py` example for AsyncLLM streaming in python (#21763)

Signed-off-by: mgoin <mgoin64@gmail.com>

examples/offline_inference/async_llm_streaming.py

+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+"""
+Simple example demonstrating streaming offline inference with AsyncLLM (V1 engine).
+
+This script shows the core functionality of vLLM's AsyncLLM engine for streaming
+token-by-token output in offline inference scenarios. It demonstrates DELTA mode
+streaming where you receive new tokens as they are generated.
+
+Usage:
+    python examples/offline_inference/async_llm_streaming.py
+"""
+
+import asyncio
+
+from vllm import SamplingParams
+from vllm.engine.arg_utils import AsyncEngineArgs
+from vllm.sampling_params import RequestOutputKind
+from vllm.v1.engine.async_llm import AsyncLLM
+
+
+async def stream_response(engine: AsyncLLM, prompt: str, request_id: str) -> None:
+    """
+    Stream response from AsyncLLM and display tokens as they arrive.
+
+    This function demonstrates the core streaming pattern:
+    1. Create SamplingParams with DELTA output kind
+    2. Call engine.generate() and iterate over the async generator
+    3. Print new tokens as they arrive
+    4. Handle the finished flag to know when generation is complete
+    """
+    print(f"\n🚀 Prompt: {prompt!r}")
+    print("💬 Response: ", end="", flush=True)
+
+    # Configure sampling parameters for streaming
+    sampling_params = SamplingParams(
+        max_tokens=100,
+        temperature=0.8,
+        top_p=0.95,
+        seed=42,  # For reproducible results
+        output_kind=RequestOutputKind.DELTA,  # Get only new tokens each iteration
+    )
+
+    try:
+        # Stream tokens from AsyncLLM
+        async for output in engine.generate(
+            request_id=request_id, prompt=prompt, sampling_params=sampling_params
+        ):
+            # Process each completion in the output
+            for completion in output.outputs:
+                # In DELTA mode, we get only new tokens generated since last iteration
+                new_text = completion.text
+                if new_text:
+                    print(new_text, end="", flush=True)
+
+            # Check if generation is finished
+            if output.finished:
+                print("\n✅ Generation complete!")
+                break
+
+    except Exception as e:
+        print(f"\n❌ Error during streaming: {e}")
+        raise
+
+
+async def main():
+    print("🔧 Initializing AsyncLLM...")
+
+    # Create AsyncLLM engine with simple configuration
+    engine_args = AsyncEngineArgs(
+        model="meta-llama/Llama-3.2-1B-Instruct",
+        enforce_eager=True,  # Faster startup for examples
+    )
+    engine = AsyncLLM.from_engine_args(engine_args)
+
+    try:
+        # Example prompts to demonstrate streaming
+        prompts = [
+            "The future of artificial intelligence is",
+            "In a galaxy far, far away",
+            "The key to happiness is",
+        ]
+
+        print(f"🎯 Running {len(prompts)} streaming examples...")
+
+        # Process each prompt
+        for i, prompt in enumerate(prompts, 1):
+            print(f"\n{'=' * 60}")
+            print(f"Example {i}/{len(prompts)}")
+            print(f"{'=' * 60}")
+
+            request_id = f"stream-example-{i}"
+            await stream_response(engine, prompt, request_id)
+
+            # Brief pause between examples
+            if i < len(prompts):
+                await asyncio.sleep(0.5)
+
+        print("\n🎉 All streaming examples completed!")
+
+    finally:
+        # Always clean up the engine
+        print("🔧 Shutting down engine...")
+        engine.shutdown()
+
+
+if __name__ == "__main__":
+    try:
+        asyncio.run(main())
+    except KeyboardInterrupt:
+        print("\n🛑 Interrupted by user")