[Ray] Improve documentation on batch inference (#16609)

Signed-off-by: Richard Liaw <rliaw@berkeley.edu>

Author: richardliaw

examples/offline_inference/batch_llm_inference.py

@@ -0,0 +1,90 @@
+# SPDX-License-Identifier: Apache-2.0
+"""
+This example shows how to use Ray Data for data parallel batch inference.
+
+Ray Data is a data processing framework that can handle large datasets
+and integrates tightly with vLLM for data-parallel inference.
+
+As of Ray 2.44, Ray Data has a native integration with
+vLLM (under ray.data.llm).
+
+Ray Data provides functionality for:
+* Reading and writing to cloud storage (S3, GCS, etc.)
+* Automatic sharding and load-balancing across a cluster
+* Optimized configuration of vLLM using continuous batching
+* Compatible with tensor/pipeline parallel inference as well.
+
+Learn more about Ray Data's LLM integration:
+https://docs.ray.io/en/latest/data/working-with-llms.html
+"""
+import ray
+from packaging.version import Version
+from ray.data.llm import build_llm_processor, vLLMEngineProcessorConfig
+
+assert Version(ray.__version__) >= Version(
+    "2.44.1"), "Ray version must be at least 2.44.1"
+
+# Uncomment to reduce clutter in stdout
+# ray.init(log_to_driver=False)
+# ray.data.DataContext.get_current().enable_progress_bars = False
+
+# 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")
+print(ds.schema())
+
+size = ds.count()
+print(f"Size of dataset: {size} prompts")
+
+# Configure vLLM engine.
+config = vLLMEngineProcessorConfig(
+    model_source="unsloth/Llama-3.1-8B-Instruct",
+    engine_kwargs={
+        "enable_chunked_prefill": True,
+        "max_num_batched_tokens": 4096,
+        "max_model_len": 16384,
+    },
+    concurrency=1,  # set the number of parallel vLLM replicas
+    batch_size=64,
+)
+
+# Create a Processor object, which will be used to
+# do batch inference on the dataset
+vllm_processor = build_llm_processor(
+    config,
+    preprocess=lambda row: dict(
+        messages=[{
+            "role": "system",
+            "content": "You are a bot that responds with haikus."
+        }, {
+            "role": "user",
+            "content": row["text"]
+        }],
+        sampling_params=dict(
+            temperature=0.3,
+            max_tokens=250,
+        )),
+    postprocess=lambda row: dict(
+        answer=row["generated_text"],
+        **row  # This will return all the original columns in the dataset.
+    ),
+)
+
+ds = vllm_processor(ds)
+
+# 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}")
+    print(f"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>")