pycnn_viz: put expression_name at the top of the node label
pycnn_viz: put expression_name at the top of the node label

Former-commit-id: b4512b27a802374ea491df51c988b27124860c64
clab/dynet,shuheik/dynet,danielhers/dynet,shuheik/dynet,clab/dynet,shuheik/dynet,danielhers/dynet,chunyang-wen/dynet,shuheik/dynet,xunzhang/dynet,chunyang-wen/dynet,clab/dynet,danielhers/dynet,clab/dynet,clab/cnn,clab/cnn,clab/dynet,xunzhang/dynet,danielhers/dynet,clab/cnn,danielhers/dynet,chunyang-wen/dynet,xunzhang/dynet,shuheik/dynet,danielhers/dynet,chunyang-wen/dynet,xunzhang/dynet,clab/dynet,shuheik/dynet,danielhers/dynet,shuheik/dynet,clab/dynet