[Experimental] Prefix Caching Support (#1669)

Co-authored-by: DouHappy <2278958187@qq.com>
Co-authored-by: Zhuohan Li <zhuohan123@gmail.com>

examples/offline_inference_with_prefix.py

@@ -0,0 +1,51 @@
+from vllm import LLM, SamplingParams
+
+prefix = (
+    "You are an expert school principal, skilled in effectively managing "
+    "faculty and staff. Draft 10-15 questions for a potential first grade "
+    "Head Teacher for my K-12, all-girls', independent school that emphasizes "
+    "community, joyful discovery, and life-long learning. The candidate is "
+    "coming in for a first-round panel interview for a 8th grade Math "
+    "teaching role. They have 5 years of previous teaching experience "
+    "as an assistant teacher at a co-ed, public school with experience "
+    "in middle school math teaching. Based on these information, fulfill "
+    "the following paragraph: ")
+
+# Sample prompts.
+prompts = [
+    "Hello, my name is",
+    "The president of the United States is",
+    "The capital of France is",
+    "The future of AI is",
+]
+# Create a sampling params object.
+sampling_params = SamplingParams(temperature=0.0)
+
+# Create an LLM.
+llm = LLM(model="facebook/opt-125m")
+
+generating_prompts = [prefix + prompt for prompt in prompts]
+
+# Generate texts from the prompts. The output is a list of RequestOutput objects
+# that contain the prompt, generated text, and other information.
+outputs = llm.generate(generating_prompts, sampling_params)
+# Print the outputs.
+for output in outputs:
+    prompt = output.prompt
+    generated_text = output.outputs[0].text
+    print(f"Prompt: {prompt!r}, Generated text: {generated_text!r}")
+
+print("-" * 80)
+
+# -1 since the last token can change when concatenating prompts.
+prefix_pos = len(llm.llm_engine.tokenizer.encode(prefix)) - 1
+
+# Generate with prefix
+outputs = llm.generate(generating_prompts, sampling_params,
+                       prefix_pos=[prefix_pos] * len(generating_prompts))
+
+# Print the outputs. You should see the same outputs as before
+for output in outputs:
+    prompt = output.prompt
+    generated_text = output.outputs[0].text
+    print(f"Prompt: {prompt!r}, Generated text: {generated_text!r}")