fix logcumsumexp functor to properly handle infs and nans (#52947)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/52213
Nans were previously inconsistently propagated due to std::min always returning first argument if one of the args in nan
when reduction functor was called on 2 `-inf` arguments, `std::min(x,y) - std::max(x,y)` resulted in `-inf - (-inf)` = nan, even though logcumsumexp is well defined for `-inf, -inf` pair.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/52947
Reviewed By: H-Huang
Differential Revision: D26718456
Pulled By: ngimel
fbshipit-source-id: a44433889da352cc959786dd15b6361a68fcfed7
