!7618 add per_batch_map useage in api comment

Merge pull request !7618 from guozhijian/add_per_batch_map_usage_in_api

mindspore/dataset/engine/datasets.py

@@ -302,6 +302,19 @@ class Dataset:
             >>> # Create a dataset where every 100 rows is combined into a batch
             >>> # and drops the last incomplete batch if there is one.
             >>> data = data.batch(100, True)
+            >>>
+            >>> # resize image according to its batch number, if it's 5-th batch, resize to (5^2, 5^2) = (25, 25)
+            >>> def np_resize(col, batchInfo):
+            >>>     output = col.copy()
+            >>>     s = (batchInfo.get_batch_num() + 1) ** 2
+            >>>     index = 0
+            >>>     for c in col:
+            >>>         img = Image.fromarray(c.astype('uint8')).convert('RGB')
+            >>>         img = img.resize((s, s), Image.ANTIALIAS)
+            >>>         output[index] = np.array(img)
+            >>>         index += 1
+            >>>     return (output,)
+            >>> data = data.batch(batch_size=8, input_columns=["image"], per_batch_map=np_resize)
         """
         return BatchDataset(self, batch_size, drop_remainder, num_parallel_workers, per_batch_map, input_columns,
                             output_columns, column_order, pad_info)