Reduce scope of dispatching in logcumsumexp_backward (#119397)
Everything inside the `AT_DISPATCH` block is being compiled 5 times,
so it makes sense to limit it to the only line that uses `scalar_t` which is
the `numeric_limits` query.
Also a small optimization, instead of computing `grad.log()` and `(-grad).log()`
we can compute `grad.abs().log()` which is 2 pointwise ops instead of 3.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/119397
Approved by: https://github.com/lezcano, https://github.com/albanD
