Add usage example to KeyedJaggedTensor docstring

[vaishnavirao24]

This PR adds a practical usage example to the KeyedJaggedTensor docstring
to improve developer understanding of how it represents variable-length
sequences across multiple keys.

This addition provides:

• A realistic input example
• Explanation of the keys, values, and lengths
• Context on where KJT is used in recommendation systems

No code logic was changed. Pure documentation improvement.

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[vaishnavirao24]

Hi, I’ve signed the CLA and all checks have passed
This is a small documentation enhancement — happy to make any changes if needed!
Thanks for reviewing

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@TroyGarden has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[TroyGarden]

thanks @vaishnavirao24 for adding the docstring. please address the pre-commit test error