use eos_token_id if there is no pad_token_id to avoid sending None (#541)

Author: wukaixingxp

apps/grpo/main.py

@@ -266,7 +266,12 @@ async def sample(self) -> dict[str, str] | None:
 
     @endpoint
     async def pad_token(self):
-        return self._tokenizer.pad_token_id
+        # Use pad_token_id if available, otherwise use eos_token_id
+        # Llama models don't have a pad token by default
+        if self._tokenizer.pad_token_id is not None:
+            return self._tokenizer.pad_token_id
+        else:
+            return self._tokenizer.eos_token_id
 
 
 async def drop_weights(version: int):