[timm_wrapper] better handling of "Unknown model" exception in timm (#40951)

* fix(timm): Add exception handling for unknown Gemma3n model

* nit: Let’s cater to this specific issue

* nit: Simplify error handling

Author: harshaljanjani

src/transformers/models/timm_wrapper/modeling_timm_wrapper.py

@@ -55,6 +55,28 @@ class TimmWrapperModelOutput(ModelOutput):
     attentions: Optional[tuple[torch.FloatTensor, ...]] = None
 
 
+def _create_timm_model_with_error_handling(config: "TimmWrapperConfig", **model_kwargs):
+    """
+    Creates a timm model and provides a clear error message if the model is not found,
+    suggesting a library update.
+    """
+    try:
+        model = timm.create_model(
+            config.architecture,
+            pretrained=False,
+            **model_kwargs,
+        )
+        return model
+    except RuntimeError as e:
+        if "Unknown model" in str(e):
+            # A good general check for unknown models.
+            raise ImportError(
+                f"The model architecture '{config.architecture}' is not supported in your version of timm ({timm.__version__}). "
+                "Please upgrade timm to a more recent version with `pip install -U timm`."
+            ) from e
+        raise e
+
+
 @auto_docstring
 class TimmWrapperPreTrainedModel(PreTrainedModel):
     main_input_name = "pixel_values"
@@ -138,7 +160,7 @@ def __init__(self, config: TimmWrapperConfig):
         super().__init__(config)
         # using num_classes=0 to avoid creating classification head
         extra_init_kwargs = config.model_args or {}
-        self.timm_model = timm.create_model(config.architecture, pretrained=False, num_classes=0, **extra_init_kwargs)
+        self.timm_model = _create_timm_model_with_error_handling(config, num_classes=0, **extra_init_kwargs)
         self.post_init()
 
     @auto_docstring
@@ -254,8 +276,8 @@ def __init__(self, config: TimmWrapperConfig):
             )
 
         extra_init_kwargs = config.model_args or {}
-        self.timm_model = timm.create_model(
-            config.architecture, pretrained=False, num_classes=config.num_labels, **extra_init_kwargs
+        self.timm_model = _create_timm_model_with_error_handling(
+            config, num_classes=config.num_labels, **extra_init_kwargs
         )
         self.num_labels = config.num_labels
         self.post_init()