added stage param to LightningDataModule.setup example (#7483)

Co-authored-by: Sileadim <christopher@omnius.com>

pytorch_lightning/core/datamodule.py

@@ -37,7 +37,7 @@ def __init__(self):
             def prepare_data(self):
                 # download, split, etc...
                 # only called on 1 GPU/TPU in distributed
-            def setup(self):
+            def setup(self, stage):
                 # make assignments here (val/train/test split)
                 # called on every process in DDP
             def train_dataloader(self):