fix(ci): correct notebook check logic nesting

[mesakhcienet]

Description

This reverts the commit that triggered the notebook workflow for all source code changes, which overwhelmed the limited 8-TPU runner pool. Now, notebooks will only run when .ipynb files are modified.

FIXES: b/534935957

Tests

We run on ci/cd workflow manually.

Checklist

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• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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