fix: session start fixture correctly handles being launched by mpirun

[terrykong]

What does this PR do ?

This fix was only in dev, so cherry-picking it into main

Changelog

• Please update the CHANGELOG.md under next version with high level changes in this PR.

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• Does the trainer resume and restore model state all states?
• Does the trainer support all parallelism techniques(PP, TP, DP)?
• Does the trainer support max_steps=-1 and validation?
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Additional Information

• Related to # (issue)