added missing "import torch" to LTB Transforms (#1584)

fixes issue #1548

Author: holly1238

beginner_source/basics/transforms_tutorial.py

@@ -26,6 +26,7 @@
 To make these transformations, we use ``ToTensor`` and ``Lambda``.
 """
 
+import torch
 from torchvision import datasets
 from torchvision.transforms import ToTensor, Lambda