runtimes: add restricted PSS security contexts

[danish9039]

What this PR does / why we need it:

This PR makes the Torch distributed Trainer runtime manifests and matching Helm templates restricted-PSS compatible.

It adds pod and container security contexts for the Torch distributed runtime and the data-cache variant, and updates Helm unit tests to cover those fields.

The data-cache dataset-initializer keeps the image default working directory because the image entrypoint runs python -m pkg.initializers.dataset from its /app module root. The training node containers keep workingDir: /tmp for non-root runtime compatibility.

Validation:

• make helm-unittest passed: 18 suites / 109 tests
• kustomize build manifests/base/runtimes
• kustomize build manifests/base/runtimes/data-cache
• helm template from a temporary chart copy after dependency build
• git diff --check

Which issue(s) this PR fixes (optional, in Fixes #<issue number>, #<issue number>, ... format, will close the issue(s) when PR gets merged):
Related to kubeflow/community-distribution#3487

Checklist:

• Docs included if any changes are user facing

[github-actions]

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[google-oss-prow]

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