[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/training_args.py

@@ -207,8 +207,8 @@ class TrainingArguments:
               :obj:`"eval_loss"`.
             - :obj:`False` if :obj:`metric_for_best_model` is not set, or set to :obj:`"loss"` or :obj:`"eval_loss"`.
         model_parallel (:obj:`bool`, `optional`, defaults to :obj:`False`):
-            If there is more than one device, whether to use model parallelism to distribute the model's modules across
-            devices or not.
+            If the model supports model parallelism and there is more than one device, whether to use model parallelism
+            to distribute the model's modules across devices or not.
         ignore_skip_data (:obj:`bool`, `optional`, defaults to :obj:`False`):
             When resuming training, whether or not to skip the epochs and batches to get the data loading at the same
             stage as in the previous training. If set to :obj:`True`, the training will begin faster (as that skipping