[NFC] polish colossalai/amp/naive_amp/__init__.py code style (#1905)

colossalai/amp/naive_amp/__init__.py

@@ -1,10 +1,13 @@
 import inspect
+
 import torch.nn as nn
 from torch.optim import Optimizer
+
 from colossalai.utils import is_no_pp_or_last_stage
-from .naive_amp import NaiveAMPOptimizer, NaiveAMPModel
-from .grad_scaler import DynamicGradScaler, ConstantGradScaler
+
 from ._fp16_optimizer import FP16Optimizer
+from .grad_scaler import ConstantGradScaler, DynamicGradScaler
+from .naive_amp import NaiveAMPModel, NaiveAMPOptimizer
 
 
 def convert_to_naive_amp(model: nn.Module, optimizer: Optimizer, amp_config):