[Doc] Add documentation for Structured Outputs (#9943)

Signed-off-by: ismael-dm <ismaeldm99@gmail.com>

examples/openai_chat_completion_structured_outputs.py

@@ -0,0 +1,94 @@
+from enum import Enum
+
+from openai import OpenAI
+from pydantic import BaseModel
+
+client = OpenAI(
+    base_url="http://localhost:8000/v1",
+    api_key="-",
+)
+
+# Guided decoding by Choice (list of possible options)
+completion = client.chat.completions.create(
+    model="Qwen/Qwen2.5-3B-Instruct",
+    messages=[{
+        "role": "user",
+        "content": "Classify this sentiment: vLLM is wonderful!"
+    }],
+    extra_body={"guided_choice": ["positive", "negative"]},
+)
+print(completion.choices[0].message.content)
+
+# Guided decoding by Regex
+prompt = ("Generate an email address for Alan Turing, who works in Enigma."
+          "End in .com and new line. Example result:"
+          "alan.turing@enigma.com\n")
+
+completion = client.chat.completions.create(
+    model="Qwen/Qwen2.5-3B-Instruct",
+    messages=[{
+        "role": "user",
+        "content": prompt,
+    }],
+    extra_body={
+        "guided_regex": "\w+@\w+\.com\n",
+        "stop": ["\n"]
+    },
+)
+print(completion.choices[0].message.content)
+
+
+# Guided decoding by JSON using Pydantic schema
+class CarType(str, Enum):
+    sedan = "sedan"
+    suv = "SUV"
+    truck = "Truck"
+    coupe = "Coupe"
+
+
+class CarDescription(BaseModel):
+    brand: str
+    model: str
+    car_type: CarType
+
+
+json_schema = CarDescription.model_json_schema()
+
+prompt = ("Generate a JSON with the brand, model and car_type of"
+          "the most iconic car from the 90's")
+completion = client.chat.completions.create(
+    model="Qwen/Qwen2.5-3B-Instruct",
+    messages=[{
+        "role": "user",
+        "content": prompt,
+    }],
+    extra_body={"guided_json": json_schema},
+)
+print(completion.choices[0].message.content)
+
+# Guided decoding by Grammar
+simplified_sql_grammar = """
+    ?start: select_statement
+
+    ?select_statement: "SELECT " column_list " FROM " table_name
+
+    ?column_list: column_name ("," column_name)*
+
+    ?table_name: identifier
+
+    ?column_name: identifier
+
+    ?identifier: /[a-zA-Z_][a-zA-Z0-9_]*/
+"""
+
+prompt = ("Generate an SQL query to show the 'username' and 'email'"
+          "from the 'users' table.")
+completion = client.chat.completions.create(
+    model="Qwen/Qwen2.5-3B-Instruct",
+    messages=[{
+        "role": "user",
+        "content": prompt,
+    }],
+    extra_body={"guided_grammar": simplified_sql_grammar},
+)
+print(completion.choices[0].message.content)