* update build_gcn.py tutorial
updates
* support bias in GCN layer
* download pretrained gcn model
* verify model accuracy
* use time_evaluator to measure runtime
* fix adding bias in gcn layer
* remove printing output
* fix small bug
* add DGL-PyTorch comparison into the build_gcn tutorial
* add accuracy testing
* adjust import order
* handle different dgl versions
* update number for dgl version checking
