fix GradScaler pre-commit (#699)

Summary:
Pull Request resolved: #699

Fixing failed pre-commit message here: {F1452790112}

Reviewed By: gunchu

Differential Revision: D53456990

fbshipit-source-id: 7b3b87f51aa16ac4eeb76ada8fbf755a55ac2967

torchtnt/framework/auto_unit.py

@@ -13,7 +13,6 @@
 
 import torch
 from pyre_extensions import none_throws
-from torch.cuda.amp import GradScaler
 from torch.distributed.fsdp import FullyShardedDataParallel as FSDP
 from torch.nn.parallel import DistributedDataParallel as DDP
 from torch.optim.swa_utils import SWALR