repair some format problem in API

mindspore/dataset/engine/datasets.py

@@ -590,7 +590,7 @@ class Dataset:
         # here again
         dataset_size = self.get_dataset_size()
 
-        if(dataset_size is None or dataset_size <= 0):
+        if dataset_size is None or dataset_size <= 0:
             raise RuntimeError("dataset size unknown, unable to split.")
 
         all_int = all(isinstance(item, int) for item in sizes)
@@ -1173,6 +1173,7 @@ class SourceDataset(Dataset):
     def is_sharded(self):
         raise NotImplementedError("SourceDataset must implement is_sharded.")
 
+
 class MappableDataset(SourceDataset):
     """
     Abstract class to represent a source dataset which supports use of samplers.