install horovod

[haolsun]

sudo LD_LIBRARY_PATH=$LD_LIBRARY_PATH HOROVOD_GPU_ALLREDUCE=NCCL HOROVOD_WITH_TENSORFLOW=1 pip3 install --no-cache-dir horovod

Horovod troubleshooting:
https://github.com/horovod/horovod/blob/master/docs/troubleshooting.md