Update dynet_viz.py
Update dynet_viz.py

Former-commit-id: e1d11dbe58e76047388e8b4008aef594d0524e09
clab/dynet,shuheik/dynet,danielhers/dynet,chunyang-wen/dynet,clab/cnn,clab/cnn,danielhers/dynet,clab/dynet,chunyang-wen/dynet,clab/dynet,xunzhang/dynet,danielhers/dynet,shuheik/dynet,xunzhang/dynet,shuheik/dynet,danielhers/dynet,danielhers/dynet,danielhers/dynet,clab/dynet,shuheik/dynet,danielhers/dynet,clab/dynet,shuheik/dynet,clab/dynet,xunzhang/dynet,clab/cnn,chunyang-wen/dynet,chunyang-wen/dynet,shuheik/dynet,clab/dynet,shuheik/dynet,xunzhang/dynet