Replace log(1 + x) with log1p(x) (#92114)
`log1p` offers better precision near zero since `(1 + x) - 1` truncates any
values less than the float epsilon to zero. For `soft_margin_loss` this also
requires one fewer kernel invocation which for numel=1e7 gives me a 1.2x speedup
on CUDA and a 1.1x speedup on CPU.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/92114
Approved by: https://github.com/ngimel, https://github.com/lezcano
