Make `kl_div` a composite function. (#80334)
Benchmarks: https://github.com/pytorch/pytorch/pull/80334#issuecomment-1167229285
Fixes https://github.com/pytorch/pytorch/issues/80158
Fixes https://github.com/pytorch/pytorch/issues/78867
Fixes https://github.com/pytorch/pytorch/issues/69230
Supersedes https://github.com/pytorch/pytorch/pull/79007
Supersedes https://github.com/pytorch/pytorch/pull/69212
Supersedes https://github.com/pytorch/pytorch/pull/19659
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80334
Approved by: https://github.com/ezyang
