Add one example to run batch inference distributed on Ray (#2696)

examples/offline_inference_distributed.py

+"""
+This example shows how to use Ray Data for running offline batch inference
+distributively on a multi-nodes cluster.
+
+Learn more about Ray Data in https://docs.ray.io/en/latest/data/data.html
+"""
+
+from vllm import LLM, SamplingParams
+from typing import Dict
+import numpy as np
+import ray
+
+# Create a sampling params object.
+sampling_params = SamplingParams(temperature=0.8, top_p=0.95)
+
+
+# Create a class to do batch inference.
+class LLMPredictor:
+
+    def __init__(self):
+        # Create an LLM.
+        self.llm = LLM(model="meta-llama/Llama-2-7b-chat-hf")
+
+    def __call__(self, batch: Dict[str, np.ndarray]) -> Dict[str, list]:
+        # Generate texts from the prompts.
+        # The output is a list of RequestOutput objects that contain the prompt,
+        # generated text, and other information.
+        outputs = self.llm.generate(batch["text"], sampling_params)
+        prompt = []
+        generated_text = []
+        for output in outputs:
+            prompt.append(output.prompt)
+            generated_text.append(' '.join([o.text for o in output.outputs]))
+        return {
+            "prompt": prompt,
+            "generated_text": generated_text,
+        }
+
+
+# Read one text file from S3. Ray Data supports reading multiple files
+# from cloud storage (such as JSONL, Parquet, CSV, binary format).
+ds = ray.data.read_text("s3://anonymous@air-example-data/prompts.txt")
+
+# Apply batch inference for all input data.
+ds = ds.map_batches(
+    LLMPredictor,
+    # Set the concurrency to the number of LLM instances.
+    concurrency=10,
+    # Specify the number of GPUs required per LLM instance.
+    # NOTE: Do NOT set `num_gpus` when using vLLM with tensor-parallelism
+    # (i.e., `tensor_parallel_size`).
+    num_gpus=1,
+    # Specify the batch size for inference.
+    batch_size=32,
+)
+
+# Peek first 10 results.
+# NOTE: This is for local testing and debugging. For production use case,
+# one should write full result out as shown below.
+outputs = ds.take(limit=10)
+for output in outputs:
+    prompt = output["prompt"]
+    generated_text = output["generated_text"]
+    print(f"Prompt: {prompt!r}, Generated text: {generated_text!r}")
+
+# Write inference output data out as Parquet files to S3.
+# Multiple files would be written to the output destination,
+# and each task would write one or more files separately.
+#
+# ds.write_parquet("s3://<your-output-bucket>")