fix: allow API endpoint to return response directly (#4581)

* fix: allow API endpoint to return response directly

Signed-off-by: Frost Ming <me@frostming.com>

src/_bentoml_impl/server/app.py

@@ -14,6 +14,7 @@
 from simple_di import Provide
 from simple_di import inject
 from starlette.middleware import Middleware
+from starlette.responses import Response
 from starlette.staticfiles import StaticFiles
 
 from _bentoml_sdk import Service
@@ -30,7 +31,6 @@
     from opentelemetry.sdk.trace import Span
     from starlette.applications import Starlette
     from starlette.requests import Request
-    from starlette.responses import Response
     from starlette.routing import BaseRoute
 
     from bentoml._internal import external_typing as ext
@@ -469,7 +469,10 @@ async def inner() -> t.AsyncGenerator[t.Any, None]:
         else:
             output = await self._to_thread(func, *input_args, **input_params)
 
-        response = await method.output_spec.to_http_response(output, serde)
+        if isinstance(output, Response):
+            response = output
+        else:
+            response = await method.output_spec.to_http_response(output, serde)
         response.headers.update({"Server": f"BentoML Service/{self.service.name}"})
 
         if method.ctx_param is not None: