Implement logaddexp (#38384)
Summary:
Resolve https://github.com/pytorch/pytorch/issues/38377
Related https://github.com/pytorch/pytorch/issues/38349
This op should be disambiguated with `logsumexp` which do a reduction on a tensor over a specific axis.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38384
Differential Revision: D21737336
Pulled By: mruberry
fbshipit-source-id: 7864d04ca304c0fb2937bb083583e3e3d6ef205d
