[Doc] Move image in dataset User guide to s3 server (#2033)

* 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

* move image to s3 server.

docs/source/guide/data.rst

@@ -25,9 +25,8 @@ To process a graph dataset located in a remote server or local disk, we
 define a class, say ``MyDataset``, inherits from :class:`dgl.data.DGLDataset`. The
 template of ``MyDataset`` is as follows.
 
-.. figure:: assets/data_flow_chart.png
+.. figure:: https://data.dgl.ai/asset/image/userguide_data_flow.png
     :align: center
-	:scale: 50 %
 
     Flow chart for graph data input pipeline defined in class DGLDataset.