add a "division by zero" check in chunked loss handling in kd_losses.py

[insop]

Context

What is the purpose of this PR? Is it to

• [ ] add a new feature
• [ ] fix a bug
• [ ] update tests and/or documentation
• [x] other (add a "division by zero" check in chunked loss handling in kd_losses.py)

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Changelog

What are the changes made in this PR?

• add a "division by zero" check in chunked loss handling in kd_losses.py (Issue https://github.com/pytorch/torchtune/issues/2225)

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UX

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