[Speculative decoding] Initial spec decode docs (#5400)

Author: cadedaniel

docs/source/index.rst

@@ -90,6 +90,7 @@ Documentation
    models/engine_args
    models/lora
    models/vlm
+   models/spec_decode
    models/performance
 
 .. toctree::

docs/source/models/spec_decode.rst

@@ -0,0 +1,75 @@
+.. _spec_decode:
+
+Speculative decoding in vLLM
+============================
+
+.. warning::
+    Please note that speculative decoding in vLLM is not yet optimized and does
+    not usually yield inter-token latency reductions for all prompt datasets or sampling parameters. The work
+    to optimize it is ongoing and can be followed in `this issue. <https://github.com/vllm-project/vllm/issues/4630>`_
+
+This document shows how to use `Speculative Decoding <https://x.com/karpathy/status/1697318534555336961>`_ with vLLM.
+Speculative decoding is a technique which improves inter-token latency in memory-bound LLM inference.
+
+Speculating with a draft model
+------------------------------
+
+The following code configures vLLM to use speculative decoding with a draft model, speculating 5 tokens at a time.
+
+.. code-block:: python
+    from vllm import LLM, SamplingParams
+    
+    prompts = [
+        "The future of AI is",
+    ]
+    sampling_params = SamplingParams(temperature=0.8, top_p=0.95)
+    
+    llm = LLM(
+        model="facebook/opt-6.7b",
+        tensor_parallel_size=1,
+        speculative_model="facebook/opt-125m",
+        num_speculative_tokens=5,
+        use_v2_block_manager=True,
+    )
+    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}")
+
+Speculating by matching n-grams in the prompt
+---------------------------------------------
+
+The following code configures vLLM to use speculative decoding where proposals are generated by
+matching n-grams in the prompt. For more information read `this thread. <https://x.com/joao_gante/status/1747322413006643259>`_
+
+.. code-block:: python
+    from vllm import LLM, SamplingParams
+    
+    prompts = [
+        "The future of AI is",
+    ]
+    sampling_params = SamplingParams(temperature=0.8, top_p=0.95)
+    
+    llm = LLM(
+        model="facebook/opt-6.7b",
+        tensor_parallel_size=1,
+        speculative_model="[ngram]",
+        num_speculative_tokens=5,
+        ngram_prompt_lookup_max=4,
+        use_v2_block_manager=True,
+    )
+    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}")
+
+Resources for vLLM contributors
+-------------------------------
+* `A Hacker's Guide to Speculative Decoding in vLLM <https://www.youtube.com/watch?v=9wNAgpX6z_4>`_
+* `What is Lookahead Scheduling in vLLM? <https://docs.google.com/document/d/1Z9TvqzzBPnh5WHcRwjvK2UEeFeq5zMZb5mFE8jR0HCs/edit#heading=h.1fjfb0donq5a>`_
+* `Information on batch expansion. <https://docs.google.com/document/d/1T-JaS2T1NRfdP51qzqpyakoCXxSXTtORppiwaj5asxA/edit#heading=h.kk7dq05lc6q8>`_
+* `Dynamic speculative decoding <https://github.com/vllm-project/vllm/issues/4565>`_