Support for negative weight in node2vec

[CHIANGEL]

For generating node embeddings using node2vec, I use the official python-version code: https://github.com/aditya-grover/node2vec

But I was caught in errors that node2vec can not support negative edge weights because there are normalization operations over weights. It turns out that, instead of the python version, we should use C++ node2vec tools which support negative weights.

So I submit this issue as a reminder.