Raise error when quantizing a quantized model (#27500)

add error msg

src/transformers/modeling_utils.py

@@ -2781,7 +2781,12 @@ class PreTrainedModel(nn.Module, ModuleUtilsMixin, GenerationMixin, PushToHubMix
             quantization_method_from_config = config.quantization_config.get(
                 "quant_method", QuantizationMethod.BITS_AND_BYTES
             )
-
+        if quantization_method_from_config is not None and quantization_method_from_args is not None:
+            if quantization_method_from_config != quantization_method_from_args:
+                raise ValueError(
+                    f"The model is already quantized with {quantization_method_from_config}. "
+                    f"You can't quantize it again with {quantization_method_from_args}"
+                )
         if quantization_method_from_config == QuantizationMethod.GPTQ and quantization_method_from_args is not None:
             loading_attr_dict = quantization_config.get_loading_attributes()
             for attr, val in loading_attr_dict.items():