[NFC] polish colossalai/amp/naive_amp/grad_scaler/dynamic_grad_scaler.py code style (#2299)

Co-authored-by: henryqin1997 <henryqin1997@gamil.com>

colossalai/amp/naive_amp/grad_scaler/dynamic_grad_scaler.py

 #!/usr/bin/env python
 # -*- encoding: utf-8 -*-
 
+from typing import Optional
+
 import torch
+
 from .base_grad_scaler import BaseGradScaler
-from typing import Optional
 
 __all__ = ['DynamicGradScaler']