[Doc] Update README.md for pytorch PinSage example. (#770)

Add noting that the PinSage model example under
example/pytorch/recommendation only work with Python 3.6+
as its dataset loader depends on stanfordnlp package
which work only with Python 3.6+.

examples/pytorch/recommendation/README.md

 # PinSage model
 
 NOTE: this version is not using NodeFlow yet.
+      
+This example only work with Python 3.6+
 
 First, download and extract from https://dgl.ai.s3.us-east-2.amazonaws.com/dataset/ml-1m.tar.gz
 

examples/pytorch/recommendation/main.py

@@ -7,7 +7,6 @@ import tqdm
 from rec.model.pinsage import PinSage
 from rec.datasets.movielens import MovieLens
 from rec.utils import cuda
-from rec.adabound import AdaBound
 from dgl import DGLGraph
 
 import argparse