* add sagpool example for pytorch backend
* polish sagpool example for pytorch backend
* [Example] SAGPool: use std variance
* [Example] SAGPool: change to std
* add sagpool example to index page
* add graph property prediction tag to sagpool
* [Example] add graph classification example HGP-SL
* [Example] fix sagpool
* fix bug
* [Example] change tab to space in README of hgp-sl
* remove redundant files
* remote redundant network
* [Example]: change link from code to doc in HGP-SL
* [Example] in HGP-SL, change to meaningful name
* [Example] Fix path mistake for 'hardgat'
Co-authored-by: zhangtianqi <tianqizh@amazon.com>
Lourens Touwen