[Misc] Auto detect bitsandbytes pre-quantized models (#16027)

Signed-off-by: Tristan Leclercq <tristanleclercq@gmail.com>

docs/source/features/quantization/bnb.md

@@ -19,17 +19,20 @@ And usually, these repositories have a config.json file that includes a quantiza
 
 ## Read quantized checkpoint
 
+For pre-quantized checkpoints, vLLM will try to infer the quantization method from the config file, so you don't need to explicitly specify the quantization argument.
+
 ```python
 from vllm import LLM
 import torch
 # unsloth/tinyllama-bnb-4bit is a pre-quantized checkpoint.
 model_id = "unsloth/tinyllama-bnb-4bit"
-llm = LLM(model=model_id, dtype=torch.bfloat16, trust_remote_code=True, \
-quantization="bitsandbytes")
+llm = LLM(model=model_id, dtype=torch.bfloat16, trust_remote_code=True)
 ```
 
 ## Inflight quantization: load as 4bit quantization
 
+For inflight 4bit quantization with BitsAndBytes, you need to explicitly specify the quantization argument.
+
 ```python
 from vllm import LLM
 import torch
@@ -40,7 +43,7 @@ quantization="bitsandbytes")
 
 ## OpenAI Compatible Server
 
-Append the following to your 4bit model arguments:
+Append the following to your model arguments for 4bit inflight quantization:
 
 ```console
 --quantization bitsandbytes

tests/quantization/test_bitsandbytes.py

@@ -41,7 +41,7 @@ def test_load_4bit_bnb_model(hf_runner, vllm_runner, example_prompts,
 
     hf_model_kwargs = {"load_in_4bit": True}
     validate_generated_texts(hf_runner, vllm_runner, example_prompts[:1],
-                             model_name, hf_model_kwargs)
+                             model_name, False, hf_model_kwargs)
 
 
 @pytest.mark.skipif(not is_quant_method_supported("bitsandbytes"),
@@ -53,7 +53,7 @@ def test_load_pre_quant_4bit_bnb_model(hf_runner, vllm_runner, example_prompts,
                                        model_name, description) -> None:
 
     validate_generated_texts(hf_runner, vllm_runner, example_prompts[:1],
-                             model_name)
+                             model_name, True)
 
 
 @pytest.mark.skipif(not is_quant_method_supported("bitsandbytes"),
@@ -65,7 +65,7 @@ def test_load_8bit_bnb_model(hf_runner, vllm_runner, example_prompts,
                              model_name, description) -> None:
 
     validate_generated_texts(hf_runner, vllm_runner, example_prompts[:1],
-                             model_name)
+                             model_name, True)
 
 
 @pytest.mark.skipif(torch.cuda.device_count() < 2,
@@ -82,6 +82,7 @@ def test_load_tp_4bit_bnb_model(hf_runner, vllm_runner, example_prompts,
                              vllm_runner,
                              example_prompts[:1],
                              model_name,
+                             False,
                              hf_model_kwargs,
                              vllm_tp_size=2)
 
@@ -128,13 +129,14 @@ def validate_generated_texts(hf_runner,
                              vllm_runner,
                              prompts,
                              model_name,
+                             pre_quant=False,
                              hf_model_kwargs=None,
                              vllm_tp_size=1):
 
     # NOTE: run vLLM first, as it requires a clean process
     # when using distributed inference
     with vllm_runner(model_name,
-                     quantization='bitsandbytes',
+                     quantization=None if pre_quant else 'bitsandbytes',
                      tensor_parallel_size=vllm_tp_size,
                      enforce_eager=False) as llm:
         vllm_outputs = llm.generate_greedy(prompts, 8)

vllm/engine/arg_utils.py

@@ -1275,6 +1275,10 @@ class EngineArgs:
             self.model_loader_extra_config[
                 "qlora_adapter_name_or_path"] = self.qlora_adapter_name_or_path
 
+        # bitsandbytes pre-quantized model need a specific model loader
+        if model_config.quantization == "bitsandbytes":
+            self.quantization = self.load_format = "bitsandbytes"
+
         load_config = self.create_load_config()
 
         prompt_adapter_config = PromptAdapterConfig(