Pass train_dataset (required by AsyncGRPOTrainer ctor in 1.15.0)

train_lora.py

@@ -138,6 +138,7 @@ def main():
     trainer = AsyncGRPOTrainer(
         model=MODEL,
         args=config,
+        train_dataset=dataset,   # required by the ctor even with an injected worker
         processing_class=tokenizer,
         rollout_worker=worker,
         peft_config=peft_config,