[lightning] Replace deprecated DDP accelerator with ddp_find_unused_parameters_false

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: D34295318

fbshipit-source-id: 7b2245ca9c7c2900f510722b33af8d8eeda49919

examples/source_separation/lightning_train.py

@@ -425,8 +425,7 @@ def cli_main():
         max_epochs=args.epochs,
         gpus=args.num_gpu,
         num_nodes=args.num_node,
-        accelerator="ddp",
-        plugins=DDPStrategy(find_unused_parameters=False),  # make sure there is no unused params
+        strategy="ddp_find_unused_parameters_false",
         limit_train_batches=1.0,  # Useful for fast experiment
         gradient_clip_val=5.0,
         callbacks=callbacks,