chore(runtimes): Remove MPI pi Runtime

[andreyvelich]

As we discussed in the Slack thread, we can remove the mpi-distributed runtime if favour of MPI-based runtimes like DeepSpeed and MLX: https://cloud-native.slack.com/archives/C0742LDFZ4K/p1753783021564389?thread_ts=1753710956.860929&cid=C0742LDFZ4K

/hold for review

/assign @kubeflow/kubeflow-trainer-team @astefanutti

[coveralls]

Pull Request Test Coverage Report for Build 17026006406

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This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+47.8%) to 47.791%

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Totals
Change from base Build 16592909581:	47.8%
Covered Lines:	941
Relevant Lines:	1969

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💛 - Coveralls

[Electronic-Waste]

@andreyvelich Thanks for creating this!
/lgtm
/approve

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[Electronic-Waste]

/retest

[andreyvelich]

I still need to fix the E2Es, let's discuss the runtime deprecation policy before merging this: https://cloud-native.slack.com/archives/C0742LDFZ4K/p1753783715133799?thread_ts=1753710956.860929&cid=C0742LDFZ4K

[andreyvelich]

@tenzen-y @Electronic-Waste @astefanutti I fixed the E2Es.
/hold cancel

[astefanutti]

/lgtm

Thanks @andreyvelich!