Fix/warnings stacklevel mvapich runner

[nathon-lee]

MVAPICHRunner.backend_exists calls warnings.warn without stacklevel,
so Python reports the warning as coming from inside DeepSpeed rather than
the user's code. Adding stacklevel=2 fixes the reported call site.

No functional change.

[tohtana]

Hi @nathon-lee,
Thank you for the PR~ I wonder if stacklevel=2 still doesn't point the user code. Before this PR, it isdeepspeed/launcher/multinode_runner.py but it will be deepspeed/launcher/runner.py.

[nathon-lee]

Hi @nathon-lee, Thank you for the PR~ I wonder if stacklevel=2 still doesn't point the user code. Before this PR, it isdeepspeed/launcher/multinode_runner.py but it will be deepspeed/launcher/runner.py.

Hi @tohtana, you're right — stacklevel=2 still points inside DeepSpeed, and since a RuntimeError is raised right after, users won't even see the warning. This change has no real effect. Converting to draft while I reconsider. Thanks!

[sfc-gh-truwase]

@nathon-lee do you intend to keep this as draft?

[nathon-lee]

Hi, @sfc-gh-truwase
Thanks for checking! I don’t plan to keep it as draft — I’ll mark it as Ready for review.
This PR only adds stacklevel=2 to warnings.warn (no functional change), so the warning points to user call sites and is easier to debug.
If this looks good to you, could you help merge it? Thank you!

[sfc-gh-truwase]

@nathon-lee can you clarify that your reconsideration based on @tohtana comment is to merge even if there is no functional impact?

[nathon-lee]

Hi @tohtana and @sfc-gh-truwase,

Thanks for the review. After revisiting the call path, I agree with your point: stacklevel=2 does not actually point to user code in this case, so this change does not provide meaningful user-facing value.

Since the launcher also raises a RuntimeError immediately afterward, I do not think this PR is worth keeping open as-is. I will close it for now rather than keep a low-impact change under review.

Thanks again for the careful feedback. If useful, I can come back later with a more meaningful improvement to the MVAPICH error/reporting path.