remove unused random_split import from tutorial (#15716)

(cherry picked from commit b40ca0e)

docs/source-pytorch/model/train_model_basic.rst

@@ -20,7 +20,7 @@ Add the relevant imports at the top of the file
     import torch.nn.functional as F
     from torchvision import transforms
     from torchvision.datasets import MNIST
-    from torch.utils.data import DataLoader, random_split
+    from torch.utils.data import DataLoader
     import pytorch_lightning as pl
 
 ----