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

[insop]

Suggested improvement:

It would be good to add a "division by zero" check in chunked loss handling in kd_losses.py.

Context:
This is based on the discussion in PR #2094.

Potential issue:
ForwardKLWithChunkedOutputLoss does not check for division by zero, while the non-chunked version does.

[joecummings]

Great catch! We'd definitely welcome a small PR for this if you want to do it, otherwise we can try to get to it soon.

[insop]

I can make a change. @felipemello1 , let me know if you prefer to do it or have a change already.

[felipemello1]

That would be awesome. Thanks @insop :)