[NFC] polish colossalai/engine/gradient_accumulation/_gradient_accumulation.py code style (#3277)

Co-authored-by: siqi <siqi@siqis-MacBook-Pro.local>

colossalai/engine/gradient_accumulation/_gradient_accumulation.py

 #!/usr/bin/env python
 # -*- encoding: utf-8 -*-
 
-from typing import Union
+from typing import Any, Iterable, Tuple, Union
+
 import torch.nn as nn
 from torch import Tensor
-from typing import Iterable, Any, Tuple
-from colossalai.nn.optimizer import ColossalaiOptimizer
 from torch.nn.parallel.distributed import DistributedDataParallel
 from torch.optim import Optimizer
 from torch.optim.lr_scheduler import _LRScheduler
 from torch.utils.data import DataLoader
-from colossalai.utils import conditional_context
+
 from colossalai.engine import BaseGradientHandler
+from colossalai.nn.optimizer import ColossalaiOptimizer
+from colossalai.utils import conditional_context
 
 
 class GradAccumOptimizer(ColossalaiOptimizer):
-    """A wrapper for the optimizer to enable gradient accumulation by skipping the steps 
+    """A wrapper for the optimizer to enable gradient accumulation by skipping the steps
     before accumulation size is reached.
 
     Args:
@@ -161,7 +162,7 @@ class GradAccumDataloader:
 
 
 class GradAccumLrSchedulerByStep(_LRScheduler):
-    """A wrapper for the LR scheduler to enable gradient accumulation by skipping the steps 
+    """A wrapper for the LR scheduler to enable gradient accumulation by skipping the steps
     before accumulation size is reached.
 
     Args: