`kl_div`: fix for grads wrt `target`, double backward, forward-over-reverse AD support. (#79007)
Fixes https://github.com/pytorch/pytorch/issues/78867,
fixes https://github.com/pytorch/pytorch/issues/65466.
Adds forward-over-reverse AD support.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/79007
Approved by: https://github.com/soulitzer, https://github.com/jbschlosser
