[Trainer] Refactor trainer + bnb logic (huggingface#26248)

* refactor trainer + bnb logic

* remove logger.info

* oops

src/transformers/trainer.py

@@ -402,19 +402,23 @@ def __init__(
                     " to `True` to avoid any unexpected behavior such as device placement mismatching."
                 )
 
+        _is_peft_model = is_peft_available() and isinstance(model, PeftModel)
+        _is_quantized_and_base_model = getattr(model, "is_quantized", False) and not getattr(
+            model, "_hf_peft_config_loaded", False
+        )
+
         # At this stage the model is already loaded
-        if getattr(model, "is_quantized", False) and not getattr(model, "_hf_peft_config_loaded", False):
-            if getattr(model, "_is_quantized_training_enabled", False):
-                logger.info(
-                    "The model is quantized. To train this model you need to add additional modules"
-                    " inside the model such as adapters using `peft` library and freeze the model weights. Please"
-                    " check the examples in https://github.com/huggingface/peft for more details."
-                )
-            else:
-                raise ValueError(
-                    "The model you want to train is loaded in 8-bit precision.  if you want to fine-tune an 8-bit"
-                    " model, please make sure that you have installed `bitsandbytes>=0.37.0`. "
-                )
+        if _is_quantized_and_base_model and not _is_peft_model:
+            raise ValueError(
+                "You cannot perform fine-tuning on purely quantized models. Please attach trainable adapters on top of"
+                " the quantized model to correctly perform fine-tuning. Please see: https://huggingface.co/docs/transformers/peft"
+                " for more details"
+            )
+        elif _is_quantized_and_base_model and not getattr(model, "_is_quantized_training_enabled", False):
+            raise ValueError(
+                "The model you want to train is loaded in 8-bit precision.  if you want to fine-tune an 8-bit"
+                " model, please make sure that you have installed `bitsandbytes>=0.37.0`. "
+            )
 
         # Setup Sharded DDP training
         self.sharded_ddp = None