Fix data parallelism in Trainer (#9566)

* Fix data parallelism in Trainer

* Update src/transformers/training_args.py

Co-authored-by: Lysandre Debut <lysandre@huggingface.co>

Co-authored-by: Lysandre Debut <lysandre@huggingface.co>

src/transformers/training_args.py

@@ -405,7 +405,7 @@ class TrainingArguments:
         default=0.0, metadata={"help": "The label smoothing epsilon to apply (zero means no label smoothing)."}
     )
     adafactor: bool = field(default=False, metadata={"help": "Whether or not to replace Adam by Adafactor."})
-    _n_gpu: int = field(init=False, repr=False, default=0)
+    _n_gpu: int = field(init=False, repr=False, default=-1)
 
     def __post_init__(self):
         if self.disable_tqdm is None:
@@ -483,6 +483,10 @@ class TrainingArguments:
             # GPUs available in the environment, so `CUDA_VISIBLE_DEVICES=1,2` with `cuda:0`
             # will use the first GPU in that env, i.e. GPU#1
             device = torch.device("cuda:0" if torch.cuda.is_available() else "cpu")
+            # Sometimes the line in the postinit has not been run before we end up here, so just checking we're not at
+            # the default value.
+            if self._n_gpu == -1:
+                self._n_gpu = torch.cuda.device_count()
             n_gpu = self._n_gpu
         else:
             # Here, we'll use torch.distributed.

tests/test_trainer.py

@@ -29,6 +29,7 @@ from transformers.testing_utils import (
     require_sentencepiece,
     require_tokenizers,
     require_torch,
+    require_torch_multi_gpu,
     slow,
 )
 from transformers.utils.hp_naming import TrialShortNamer
@@ -374,6 +375,22 @@ class TrainerIntegrationTest(unittest.TestCase):
         new_eval_dataset = RegressionDataset(length=128)
         self.assertEqual(len(trainer.get_eval_dataloader(new_eval_dataset)), 128 // (32 * n_gpu))
 
+    @require_torch_multi_gpu
+    def test_data_is_not_parallelized_when_model_is_parallel(self):
+        model = RegressionModel()
+        # Make the Trainer believe it's a parallelized model
+        model.is_parallelizable = True
+        model.model_parallel = True
+        trainer = Trainer(model=model, train_dataset=RegressionDataset(), eval_dataset=RegressionDataset())
+        # Check the Trainer was fooled
+        self.assertTrue(trainer.is_model_parallel)
+
+        # The batch size of the training and evaluation dataloaders should be 16, not 16 * n_gpu
+        self.assertEqual(trainer.get_train_dataloader().batch_size, 16)
+        self.assertEqual(len(trainer.get_train_dataloader()), 64 // 16)
+        self.assertEqual(trainer.get_eval_dataloader().batch_size, 16)
+        self.assertEqual(len(trainer.get_eval_dataloader()), 64 // 16)
+
     def test_evaluate(self):
         trainer = get_regression_trainer(a=1.5, b=2.5, compute_metrics=AlmostAccuracy())
         results = trainer.evaluate()