[NFC] polish ./colossalai/amp/torch_amp/__init__.py code style (#1836)

colossalai/amp/torch_amp/__init__.py

@@ -1,9 +1,12 @@
+from typing import Optional
+
 import torch.nn as nn
-from torch.optim import Optimizer
 from torch.nn.modules.loss import _Loss
+from torch.optim import Optimizer
+
 from colossalai.context import Config
-from .torch_amp import TorchAMPOptimizer, TorchAMPModel, TorchAMPLoss
+
-from typing import Optional
+from .torch_amp import TorchAMPLoss, TorchAMPModel, TorchAMPOptimizer
 
 
 def convert_to_torch_amp(model: nn.Module,