Fix style check (#2258)

Summary:
fix a style check failure from internal diff

Pull Request resolved: https://github.com/pytorch/audio/pull/2258

Reviewed By: nateanl

Differential Revision: D34459526

Pulled By: carolineechen

fbshipit-source-id: d0e6782b5689c3bf63214a4ec6a75dd757678e0d

examples/source_separation/lightning_train.py

@@ -19,7 +19,6 @@ import torch
 import torchaudio
 from pytorch_lightning import LightningModule, Trainer
 from pytorch_lightning.callbacks import ModelCheckpoint, EarlyStopping
-from pytorch_lightning.strategies import DDPStrategy
 from torch import nn
 from torch.optim.lr_scheduler import _LRScheduler
 from torch.utils.data import DataLoader