[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/trainer.py

@@ -242,6 +242,11 @@ class Trainer:
         if model is None and model_init is not None:
             model = self.call_model_init()
 
+        if self.args.model_parallel and not model.is_parallelizable:
+            raise ValueError(
+                f"{model.__class__.__name__} implementation currently doesn't support model parallelism, therefore --model_parallel cl arg cannot be used"
+            )
+
         # Model parallel
         if model is not None and not self.args.model_parallel:
             model = model.to(args.device)