https://github.com/horovod/horovod/blob/master/docs/contributors.rst
如何在Linux或Ubuntu18下安装virtualenv https://developer.aliyun.com/article/613957
https://docs.nvidia.com/deeplearning/nccl/install-guide/index.html#tar
horovod with nccl hang all the time https://github.com/NVIDIA/nccl/issues/224