[NFC] polish colossalai/amp/__init__.py code style (#3272)

colossalai/amp/__init__.py

 #!/usr/bin/env python
 # -*- encoding: utf-8 -*-
 
-from .amp_type import AMP_TYPE
-from colossalai.context import Config
 import torch.nn as nn
-from torch.optim import Optimizer
 from torch.nn.modules.loss import _Loss
-from .torch_amp import convert_to_torch_amp
+from torch.optim import Optimizer
+
+from colossalai.context import Config
+
+from .amp_type import AMP_TYPE
 from .apex_amp import convert_to_apex_amp
 from .naive_amp import convert_to_naive_amp
+from .torch_amp import convert_to_torch_amp
 
 __all__ = ['convert_to_amp', 'convert_to_naive_amp', 'convert_to_apex_amp', 'convert_to_torch_amp', 'AMP_TYPE']