Add deprecation path for renamed training type plugins (#11227)

Summary:
### New commit log messages
  4eede7c30 Add deprecation path for renamed training type plugins (#11227)

Reviewed By: edward-io, daniellepintz

Differential Revision: D33409991

fbshipit-source-id: 373e48767e992d67db3c85e436648481ad16c9d0

tools/lightning_train_net.py

@@ -25,7 +25,7 @@ from pytorch_lightning.loggers import TensorBoardLogger
 from torch.distributed import get_rank
 
 try:
-    from pytorch_lightning.plugins import DDPStrategy
+    from pytorch_lightning.strategies import DDPStrategy
 except ImportError:
     assert os.getenv("OSSRUN") == "1"
     # FIXME: DDPStrategy has been renamed to DDPStrategy, however internal version is