[Doc] Dataset api doc add save_info(), load_info() (#1954)

* PPIDataset

* Revert "PPIDataset"

This reverts commit 264bd0c960cfa698a7bb946dad132bf52c2d0c8a.

* update doc

* Update data.rst
Co-authored-by: Jinjing Zhou <VoVAllen@users.noreply.github.com>
Co-authored-by: xiang song(charlie.song) <classicxsong@gmail.com>

docs/source/api/python/data.rst

@@ -193,6 +193,8 @@ Utils
     utils.save_graphs
     utils.load_graphs
     utils.load_labels
+    utils.save_info
+    utils.load_info
 
 .. autoclass:: dgl.data.utils.Subset
     :members: __getitem__, __len__