Add chat doc in quick start (vllm-project#21213)

Co-authored-by: Cyrus Leung <cyrus.tl.leung@gmail.com>
Signed-off-by: Diego-Castan <diego.castan@ibm.com>

Author: 2 people

docs/getting_started/quickstart.md

@@ -98,6 +98,43 @@ for output in outputs:
     print(f"Prompt: {prompt!r}, Generated text: {generated_text!r}")
 ```
 
+!!! note
+    The `llm.generate` method does not automatically apply the model's chat template to the input prompt. Therefore, if you are using an Instruct model or Chat model, you should manually apply the corresponding chat template to ensure the expected behavior. Alternatively, you can use the `llm.chat` method and pass a list of messages which have the same format as those passed to OpenAI's `client.chat.completions`:
+
+    ??? code
+    
+        ```python
+        # Using tokenizer to apply chat template
+        from transformers import AutoTokenizer
+    
+        tokenizer = AutoTokenizer.from_pretrained("/path/to/chat_model")
+        messages_list = [
+            [{"role": "user", "content": prompt}]
+            for prompt in prompts
+        ]
+        texts = tokenizer.apply_chat_template(
+            messages_list,
+            tokenize=False,
+            add_generation_prompt=True,
+        )
+        
+        # Generate outputs
+        outputs = llm.generate(texts, 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}")
+    
+        # Using chat interface.
+        outputs = llm.chat(messages_list, sampling_params)
+        for idx, output in enumerate(outputs):
+            prompt = prompts[idx]
+            generated_text = output.outputs[0].text
+            print(f"Prompt: {prompt!r}, Generated text: {generated_text!r}")
+        ```
+
 [](){ #quickstart-online }
 
 ## OpenAI-Compatible Server