[Documentation] No logsumexp in NNlib page

[JinraeKim]

In NNlib page, I cannot find a documentation of logsumexp, while it's written here.

I think it would be better to expose this function explicitly.

[JinraeKim]

One more: why is it numerically stable? Can you share any references or small comments?

[DhairyaLGandhi]

Yes, that would require an addition to https://github.com/FluxML/Flux.jl/blob/master/docs/src/models/nnlib.md

I pushed out a quick PR to add it in.

https://gregorygundersen.com/blog/2020/02/09/log-sum-exp/ seems like a decent explanation of why it's stable.

[JinraeKim]

@DhairyaLGandhi Thank you!