[seq2seq testing] multigpu test run via subprocess (#7281)

Co-authored-by: Sam Shleifer <sshleifer@gmail.com>

examples/lightning_base.py

@@ -170,7 +170,7 @@ class BaseTransformer(pl.LightningModule):
             self.dataset_size = len(self.test_dataloader().dataset)
         else:
             self.train_loader = self.get_dataloader("train", self.hparams.train_batch_size, shuffle=True)
-            self.dataset_size = len(self.train_loader.dataset)
+            self.dataset_size = len(self.train_dataloader().dataset)
 
     def get_dataloader(self, type_path: str, batch_size: int, shuffle: bool = False):
         raise NotImplementedError("You must implement this for your task")