Renamed horovod.run to horovod.runner to simplify using interactive run mode

[tgaddair]

The purpose of this PR is to simplify the process of using interactive run mode and to better clarify the package structure of Horovod.

Instead of doing this:

from horovod.run.runner import run

run(...)

Users can now do this:

import horovod

horovod.run(...)

As a result, the following have been renamed:

horovod.run -> horovod.runner
horovod.run.runner.py -> horovod.runner.launch.py

[EnricoMi]

Looks good, aligns with horovod.spark.run

[alsrgv]

Nice!