pycnn_viz: fixed dim of pick nodes
pycnn_viz: fixed dim of pick nodes

Former-commit-id: 8298d5e859a5b55dc71c39c5b2d750c0826f5e18
xunzhang/dynet,danielhers/dynet,danielhers/dynet,shuheik/dynet,clab/cnn,xunzhang/dynet,shuheik/dynet,chunyang-wen/dynet,clab/dynet,xunzhang/dynet,danielhers/dynet,danielhers/dynet,danielhers/dynet,clab/dynet,clab/dynet,xunzhang/dynet,danielhers/dynet,danielhers/dynet,chunyang-wen/dynet,chunyang-wen/dynet,clab/dynet,clab/cnn,clab/dynet,shuheik/dynet,clab/dynet,shuheik/dynet,shuheik/dynet,shuheik/dynet,shuheik/dynet,chunyang-wen/dynet,clab/cnn,clab/dynet