!13823 [MD] fix bug in closing multiprocessing pool

From: @liyong126
Reviewed-by: @heleiwang,@liucunwei
Signed-off-by: @liucunwei

mindspore/dataset/engine/datasets.py

@@ -176,6 +176,15 @@ class Dataset:
         _init_device_info()
         return ir_tree, dataset
 
+    def close_pool(self):
+        """
+        Close multiprocessing pool in dataset.
+        """
+        if hasattr(self, 'process_pool') and self.process_pool is not None:
+            self.process_pool.close()
+        for child in self.children:
+            child.close_pool()
+
     @staticmethod
     def _get_operator_id(dataset):
         """
@@ -1448,6 +1457,7 @@ class Dataset:
         if self._col_names is None:
             runtime_getter = self._init_tree_getters()
             self._col_names = runtime_getter[0].GetColumnNames()
+            self.close_pool()
         return self._col_names
 
     def output_shapes(self):
@@ -1461,6 +1471,7 @@ class Dataset:
             runtime_getter = self._init_tree_getters()
             self.saved_output_shapes = runtime_getter[0].GetOutputShapes()
             self.saved_output_types = runtime_getter[0].GetOutputTypes()
+            self.close_pool()
         return self.saved_output_shapes
 
     def output_types(self):
@@ -1474,6 +1485,7 @@ class Dataset:
             runtime_getter = self._init_tree_getters()
             self.saved_output_shapes = runtime_getter[0].GetOutputShapes()
             self.saved_output_types = runtime_getter[0].GetOutputTypes()
+            self.close_pool()
         return self.saved_output_types
 
     def get_dataset_size(self):
@@ -1486,6 +1498,7 @@ class Dataset:
         if self.dataset_size is None:
             runtime_getter = self._init_size_getter()
             self.dataset_size = runtime_getter[0].GetDatasetSize(False)
+            self.close_pool()
         return self.dataset_size
 
     def num_classes(self):
@@ -1498,6 +1511,7 @@ class Dataset:
         if self._num_classes is None:
             runtime_getter = self._init_tree_getters()
             self._num_classes = runtime_getter[0].GetNumClasses()
+            self.close_pool()
         if self._num_classes == -1:
             return None
         return self._num_classes