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As discussed in https://github.com/microsoft/DeepSpeed/issues/662 this PR modifies the doc:
* explains what to use instead of CUDA_VISIBLE_DEVICES
* puts the `--hostfile` cl arg in the correct place in the invocation script

Fixes: https://github.com/microsoft/DeepSpeed/issues/662

Co-authored-by: Jeff Rasley <jerasley@microsoft.com>