Change default "role" from "system" to "user" (#53)

This is a PR for #52. I am not sure if there are other implications but
with that change it works for Gemma 2 9b IT.

Author: philschmid

src/guidellm/backend/openai.py

@@ -106,7 +106,7 @@ async def make_request(
         stream = await self._async_client.chat.completions.create(
             model=self.model,
             messages=[
-                {"role": "system", "content": request.prompt},
+                {"role": "user", "content": request.prompt},
             ],
             stream=True,
             **request_args,

tests/unit/backend/test_openai_backend.py

@@ -171,7 +171,7 @@ async def test_openai_backend_make_request(req, request_args, mock_openai_client
     assert backend._async_client.create_args == ()  # type: ignore
     assert backend._async_client.create_kwargs["model"] == "mock-model"  # type: ignore
     assert backend._async_client.create_kwargs["messages"] == [  # type: ignore
-        {"role": "system", "content": req.prompt}
+        {"role": "user", "content": req.prompt}
     ]
     assert backend._async_client.create_kwargs["stream"]  # type: ignore
     assert backend._async_client.create_kwargs["n"] == 1  # type: ignore