Fix horovod installation base-cuda Dockerfile

[akihironitta]

What does this PR do?

Fixes a similar problem to #11596, but the issue is horovod this time.

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cc @tchaton @rohitgr7 @akihironitta @carmocca @Borda

[akihironitta]

Watching if pip list in Docker / build-CUDA (3.7, 1.8) (pull_request) shows horovod.
https://github.com/PyTorchLightning/pytorch-lightning/runs/5112572709?check_suite_focus=true

[Borda]

@akihironitta very nice, just adding sanity checks that the installed packages are available (can be imported)