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

[insop]

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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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[insop]

@felipemello1 , @joecummings , PTAL

[insop]

Thank you @ebsmothers , updated the change.