[from_pretrained] imporve the error message when `_no_split_modules` is not defined (#23861)

* Better warning

* Update src/transformers/modeling_utils.py
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* format line

---------
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

src/transformers/modeling_utils.py

@@ -2734,7 +2734,10 @@ class PreTrainedModel(nn.Module, ModuleUtilsMixin, GenerationMixin, PushToHubMix
                 target_dtype = torch.int8
 
             if model._no_split_modules is None:
-                raise ValueError(f"{model.__class__.__name__} does not support `device_map='{device_map}'` yet.")
+                raise ValueError(
+                    f"{model.__class__.__name__} does not support `device_map='{device_map}'`. To implement support, the model"
+                    "class needs to implement the `_no_split_modules` attribute."
+                )
             no_split_modules = model._no_split_modules
             if device_map not in ["auto", "balanced", "balanced_low_0", "sequential"]:
                 raise ValueError(