Fix GRPO logit scaling when model is wrapped by DDP (#5955)

---------

Co-authored-by: oobabooga <112222186+oobabooga@users.noreply.github.com>

tests/python/test_grpo_ddp_model_config.py

@@ -0,0 +1,31 @@
+"""GRPO logit-scaling helpers must read config through DDP wrappers."""
+
+from __future__ import annotations
+
+import os
+
+REPO_ROOT = os.path.abspath(os.path.join(os.path.dirname(__file__), os.pardir, os.pardir))
+SOURCE_PATH = os.path.join(REPO_ROOT, "unsloth", "models", "rl_replacements.py")
+
+
+def _read_source() -> str:
+    with open(SOURCE_PATH, "r") as fh:
+        return fh.read()
+
+
+def test_grpo_logit_scaling_uses_model_config_helper():
+    src = _read_source()
+    # Helper exists and unwraps DDP/Accelerate wrappers via `.module`.
+    assert "def _unsloth_get_model_config(model):" in src
+    assert 'getattr(model.module, "config", None)' in src
+    # Softcapping takes the model and tolerates a missing config.
+    assert "logit_softcapping = _unsloth_get_final_logit_softcapping(model)" in src
+    assert "if config is None:" in src.split("def _unsloth_get_final_logit_softcapping")[1]
+    # Logit scale/divide read through the unwrapped config, not bare model.config.
+    assert 'getattr(model_config, "logit_scale", 0)' in src
+    assert 'getattr(model_config, "logits_scaling", 0)' in src
+    assert src.count("model_config = _unsloth_get_model_config(model)") >= 2
+    # Helper source is injected into the compiled GRPO trainer.
+    assert "inspect.getsource(_unsloth_get_model_config)" in src
+    # No direct model.config access remains in the RL logit path.
+    assert "model.config" not in src