fix when dataset is GeneratorDataset, sampler is DistributedSampler, dataset_size may be error

mindspore/dataset/engine/datasets.py

@@ -3795,10 +3795,10 @@ class GeneratorDataset(MappableDataset):
         # lose attribution of '__len__' after deepcopy.
         self.dataset_size = None
         if hasattr(self.source, "__len__"):
-            if not self.num_shards:
+            if not isinstance(self.sampler, samplers.DistributedSampler):
                 self.dataset_size = len(self.source)
             else:
-                self.dataset_size = math.ceil(len(self.source) / self.num_shards)
+                self.dataset_size = math.ceil(len(self.source) / self.sampler.num_shards)
 
             rows_from_sampler = self._get_sampler_dataset_size()
             if self.num_samples is not None and self.num_samples < rows_from_sampler: