[bug-fix] supported_tasks is breaking backward compatibility at init_app_state (#34027)

Signed-off-by: Kourosh Hakhamaneshi <kourosh@anyscale.com>
Signed-off-by: kourosh hakhamaneshi <31483498+kouroshHakha@users.noreply.github.com>
Co-authored-by: Cyrus Leung <cyrus.tl.leung@gmail.com>

vllm/entrypoints/openai/api_server.py

@@ -8,6 +8,7 @@ import os
 import signal
 import socket
 import tempfile
+import warnings
 from argparse import Namespace
 from collections.abc import AsyncIterator
 from contextlib import asynccontextmanager
@@ -62,6 +63,8 @@ prometheus_multiproc_dir: tempfile.TemporaryDirectory
 # Cannot use __name__ (https://github.com/vllm-project/vllm/pull/4765)
 logger = init_logger("vllm.entrypoints.openai.api_server")
 
+_FALLBACK_SUPPORTED_TASKS: tuple[SupportedTask, ...] = ("generate",)
+
 
 @asynccontextmanager
 async def build_async_engine_client(
@@ -152,7 +155,19 @@ async def build_async_engine_client_from_engine_args(
             async_llm.shutdown()
 
 
-def build_app(args: Namespace, supported_tasks: tuple["SupportedTask", ...]) -> FastAPI:
+def build_app(
+    args: Namespace, supported_tasks: tuple["SupportedTask", ...] | None = None
+) -> FastAPI:
+    if supported_tasks is None:
+        warnings.warn(
+            "The 'supported_tasks' parameter was not provided to "
+            "build_app and will be required in a future version. "
+            "Defaulting to ('generate',).",
+            DeprecationWarning,
+            stacklevel=2,
+        )
+        supported_tasks = _FALLBACK_SUPPORTED_TASKS
+
     if args.disable_fastapi_docs:
         app = FastAPI(
             openapi_url=None, docs_url=None, redoc_url=None, lifespan=lifespan
@@ -263,9 +278,18 @@ async def init_app_state(
     engine_client: EngineClient,
     state: State,
     args: Namespace,
-    supported_tasks: tuple["SupportedTask", ...],
+    supported_tasks: tuple["SupportedTask", ...] | None = None,
 ) -> None:
     vllm_config = engine_client.vllm_config
+    if supported_tasks is None:
+        warnings.warn(
+            "The 'supported_tasks' parameter was not provided to "
+            "init_app_state and will be required in a future version. "
+            "Please pass 'supported_tasks' explicitly.",
+            DeprecationWarning,
+            stacklevel=2,
+        )
+        supported_tasks = _FALLBACK_SUPPORTED_TASKS
 
     if args.served_model_name is not None:
         served_model_names = args.served_model_name