[Doc] Modify the link of save_graphs and load_graphs in dataset user guide (#2027)

* PPIDataset

* Revert "PPIDataset"

This reverts commit 264bd0c960cfa698a7bb946dad132bf52c2d0c8a.

* data pipeline user guide

* remove chapter numbers

* Update data.rst

* image in dataset userguide

* update links using ref

* modify the link of save_graphs and load_graphs in dataset user guide

docs/source/api/python/dgl.data.rst

@@ -208,8 +208,6 @@ Utils
     utils.check_sha1
     utils.extract_archive
     utils.split_dataset
-    utils.save_graphs
-    utils.load_graphs
     utils.load_labels
     utils.save_info
     utils.load_info

docs/source/guide/data.rst

@@ -494,7 +494,7 @@ We recommend to implement saving and loading functions to cache the
 processed data in local disk. This saves a lot of data processing time
 in most cases. We provide four functions to make things simple:
 
--  :func:`dgl.data.utils.save_graphs` and :func:`dgl.data.utils.load_graphs`: save/load DGLGraph objects and labels to/from local disk.
+-  :func:`dgl.save_graphs` and :func:`dgl.load_graphs`: save/load DGLGraph objects and labels to/from local disk.
 -  :func:`dgl.data.utils.save_info` and :func:`dgl.data.utils.load_info`: save/load useful information of the dataset (python ``dict`` object) to/from local disk.
 
 The following example shows how to save and load a list of graphs and
@@ -503,8 +503,8 @@ dataset information.
 .. code:: 
 
     import os
-    from dgl.data.utils import makedirs, \
-        save_graphs, load_graphs, save_info, load_info
+    from dgl import save_graphs, load_graphs
+    from dgl.data.utils import makedirs, save_info, load_info
     
     def save(self):
         # save graphs and labels