Fix TrainingArguments to config conversion in _backwards_compatible_trainer

The isinstance guard checked against TrainingArguments, but all TRL config
classes (DPOConfig, GRPOConfig, SFTConfig, etc.) inherit from it, so the
check was always True and the conversion branch was never reached. When a
user passed plain TrainingArguments to DPOTrainer, it was forwarded as-is,
causing AttributeError on config-specific attributes like padding_value.

Check against config_class instead so plain TrainingArguments is properly
converted while already-correct configs pass through unchanged. This
preserves the GRPO/RLOO mutual-exclusivity guard since those configs are
not re-initialized.

Fixes #4155

unsloth/trainer.py

@@ -302,11 +302,14 @@ def _backwards_compatible_trainer(trainer_class, config_class):
             # Reinitialising config class with parameters (that were none initially but populated on first init)
             # causes the 2nd init to fail as there are mutual exclusive checks on pairs of parameters.
             # Refer: https://github.com/huggingface/trl/blob/main/trl/trainer/grpo_config.py#L499-L502 for example
-            # So we only create config class if the previous init was not TrainingArguments
-            if not isinstance(training_args, TrainingArguments):
-                config = config_class(**config_dict)
-            else:
+            # So we only create config class if training_args is not already the correct type.
+            # We check against config_class (not TrainingArguments) because all TRL configs
+            # (DPOConfig, GRPOConfig, etc.) are subclasses of TrainingArguments, so isinstance
+            # against TrainingArguments is always True and never triggers conversion.
+            if isinstance(training_args, config_class):
                 config = training_args
+            else:
+                config = config_class(**config_dict)
 
             # Reconstruct kwargs for Trainer
             kwargs = trainer_kwargs