don't create a reference model if grpo beta is 0.0 (#2983) [skip ci]

src/axolotl/train.py

@@ -115,8 +115,11 @@ def setup_reference_model(
             LOG.debug("Passing model_ref: None to RL trainer")
             model_ref = None  # explicit setting to None
         else:
+            reference_model: bool = True
+            if cfg.rl == RLType.GRPO and cfg.trl.beta == 0:
+                reference_model = False
             # load the model again for model_ref/baseline
-            model_loader = ModelLoader(cfg, tokenizer, reference_model=True)
+            model_loader = ModelLoader(cfg, tokenizer, reference_model=reference_model)
             model_ref, _ = model_loader.load()
     return model_ref