[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]:
         """

src/transformers/models/gpt2/modeling_gpt2.py

@@ -337,6 +337,7 @@ class GPT2PreTrainedModel(PreTrainedModel):
     config_class = GPT2Config
     load_tf_weights = load_tf_weights_in_gpt2
     base_model_prefix = "transformer"
+    is_parallelizable = True
 
     def __init__(self, *inputs, **kwargs):
         super().__init__(*inputs, **kwargs)

src/transformers/models/t5/modeling_t5.py

@@ -683,6 +683,7 @@ class T5PreTrainedModel(PreTrainedModel):
     config_class = T5Config
     load_tf_weights = load_tf_weights_in_t5
     base_model_prefix = "transformer"
+    is_parallelizable = True
 
     @property
     def dummy_inputs(self):

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)

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