feat: Add notebook for TabNet on Vertex Pipelines

[sakagarwal]

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[helinwang]

@ivanmkc could you help take a look at this as well? This is also for structured data. Thanks!

[sakagarwal]

@reznitskii I cleaned up the text in the notebook by removing the large parameters section, adding links to documentation, and adding more details under the CustomJob/HPT job sections. Could you please help take a look? Any feedback would be helpful. Thanks!