[PyTorchLightning/pytorch-lightning] Rename `DDPPlugin` to `DDPStrategy` (#11142)

Summary:
### New commit log messages
  b64dea9dc Rename `DDPPlugin` to `DDPStrategy` (#11142)

Reviewed By: jjenniferdai

Differential Revision: D33259306

fbshipit-source-id: b4608c6b96b4a7977eaa4ed3f03c4b824882aef0

examples/source_separation/lightning_train.py

@@ -19,7 +19,7 @@ import torch
 import torchaudio
 from pytorch_lightning import LightningModule, Trainer
 from pytorch_lightning.callbacks import ModelCheckpoint, EarlyStopping
-from pytorch_lightning.plugins import DDPPlugin
+from pytorch_lightning.plugins import DDPStrategy
 from torch import nn
 from torch.optim.lr_scheduler import _LRScheduler
 from torch.utils.data import DataLoader
@@ -426,7 +426,7 @@ def cli_main():
         gpus=args.num_gpu,
         num_nodes=args.num_node,
         accelerator="ddp",
-        plugins=DDPPlugin(find_unused_parameters=False),  # make sure there is no unused params
+        plugins=DDPStrategy(find_unused_parameters=False),  # make sure there is no unused params
         limit_train_batches=1.0,  # Useful for fast experiment
         gradient_clip_val=5.0,
         callbacks=callbacks,