Replace deprecated DDP accelerator with DDP strategy within stl/lightning

Summary: We proactively remove references to the deprecated DDP accelerator to prepare for the breaking changes following the release of PyTorch Lighting 1.6 (see T112240890).

Differential Revision: D34295384

fbshipit-source-id: 055244f62727ac7a2d00272c06febf117de0e764

Author: tangbinh

torchx/examples/apps/lightning_classy_vision/train.py

@@ -142,7 +142,7 @@ def main(argv: List[str]) -> None:
             num_nodes=num_nodes,
             num_processes=num_processes,
             gpus=gpus,
-            accelerator="ddp",
+            strategy="ddp",
             logger=logger,
             max_epochs=args.epochs,
             callbacks=callbacks,