message: fix horovod tests.
diff: @@ -63,7 +63,10 @@ g++ --version
  # Install Horovod.
  cd ..
  +HOROVOD_GPU_OPERATIONS=NCCL
  HOROVOD_WITH_TENSORFLOW=1
  +HOROVOD_WITHOUT_PYTORCH=1
  +HOROVOD_WITHOUT_MXNET=1
  pip3.7 install horovod[tensorflow] --user
  # Install tests.
  