Mpi mode sets all nodes to the same SM_CURRENT_HOST

[verdimrc]

Describe the bug
With mpi mode, all nodes report the same SM_CURRENT_HOST (which is the master's one).

To reproduce
Run an PyTorch estimator in mpi mode and more than one node. The training entrypoint can simply dump all its environment variables to stdout (which should end-up on Cloudwatch log). From there, we can see that SM_CURRENT_HOST from all nodes are set to the same value (i.e., the master's), whereas PMIX_HOSTNAME is set correctly.

Expected behavior
Master node should not propagate its SM_CURRENT_HOST to the other nodes.

Screenshots or logs
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System information
PyTorch DLC 1.11.0-gpu-py38

Additional context
Add any other context about the problem here.

This patch corrected the SM_CURRENT_HOST issue on my training jobs.

# https://github.com/aws/sagemaker-training-toolkit/blob/3188a9df7803798defb043a332d789f7474219d0/src/sagemaker_training/mpi.py#L353
        for name in self._env_vars:
            if name.startswith("SM_"):    # New addition
                continue                  # New addition
            command.extend(["-x", name])