[trainer] --model_parallel hasn't been implemented for most models (#9347)

* --model_parallel hasn't been implemented for most models

* make the help clear as well

* implement is_parallelizable; use it

* oops

* remove property

src/transformers/modeling_utils.py

@@ -404,6 +404,7 @@ class PreTrainedModel(nn.Module, ModuleUtilsMixin, GenerationMixin):
 
         - **base_model_prefix** (:obj:`str`) -- A string indicating the attribute associated to the base model in
           derived classes of the same architecture adding modules on top of the base model.
+        - **is_parallelizable** (:obj:`bool`) -- A flag indicating whether this model supports model parallelization.
     """
     config_class = None
     base_model_prefix = ""
@@ -417,6 +418,8 @@ class PreTrainedModel(nn.Module, ModuleUtilsMixin, GenerationMixin):
     # trained, but which are deterministic)
     _keys_to_ignore_on_save = None
 
+    is_parallelizable = False
+
     @property
     def dummy_inputs(self) -> Dict[str, torch.Tensor]:
         """