Add logsumexp to AMP autocast (#76330)
Add `logsumexp` function to AMP rules.
This PR fixes an issue described in [PyTorch forum](https://discuss.pytorch.org/t/kl-divergence-negative-with-amp/149312).
cc @ptrblck @mcarilli
Pull Request resolved: https://github.com/pytorch/pytorch/pull/76330
Approved by: https://github.com/mcarilli, https://github.com/ptrblck, https://github.com/ngimel
