Add an example for reproducibility (#15262)

Signed-off-by: Woosuk Kwon <woosuk.kwon@berkeley.edu>

examples/offline_inference/reproduciblity.py

+# SPDX-License-Identifier: Apache-2.0
+import os
+
+from vllm import LLM, SamplingParams
+
+# vLLM does not guarantee the reproducibility of the results by default,
+# for the sake of performance. You need to do the following to achieve
+# reproducible results:
+# 1. Turn off multiprocessing to make the scheduling deterministic.
+#    NOTE(woosuk): This is not needed and will be ignored for V0.
+os.environ["VLLM_ENABLE_V1_MULTIPROCESSING"] = "0"
+# 2. Fix the global seed for reproducibility. The default seed is None, which is
+# not reproducible.
+SEED = 42
+
+# NOTE(woosuk): Even with the above two settings, vLLM only provides
+# reproducibility when it runs on the same hardware and the same vLLM version.
+# Also, the online serving API (`vllm serve`) does not support reproducibility
+# because it is almost impossible to make the scheduling deterministic in the
+# online serving setting.
+
+llm = LLM(model="facebook/opt-125m", seed=SEED)
+
+prompts = [
+    "Hello, my name is",
+    "The president of the United States is",
+    "The capital of France is",
+    "The future of AI is",
+]
+sampling_params = SamplingParams(temperature=0.8, top_p=0.95)
+
+outputs = llm.generate(prompts, sampling_params)
+for output in outputs:
+    prompt = output.prompt
+    generated_text = output.outputs[0].text
+    print(f"Prompt: {prompt!r}, Generated text: {generated_text!r}")