logsoftmax with dims

[mcabbott]

For #77, this adds a version of logsoftmax which takes dims keyword, and ∇logsoftmax too.

It also adjusts the implementation for softmax from #130 so as not to exponentiate twice; this is about twice as fast, see #77 (comment) .

Both of these mutate their out array, which could easily be avoided if desired, at the cost of twice the memory.

[codecov-io]

Codecov Report

Merging #135 into master will decrease coverage by 1.29%.
The diff coverage is 88.88%.

@@            Coverage Diff            @@
##           master     #135     +/-   ##
=========================================
- Coverage   79.03%   77.73%   -1.3%     
=========================================
  Files          24       24             
  Lines         763      768      +5     
=========================================
- Hits          603      597      -6     
- Misses        160      171     +11

Impacted Files	Coverage Δ
src/softmax.jl	27.02% <88.88%> (-22.98%)	⬇️

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[MikeInnes]

It would be best to avoid in-place operations, since these are designed to be AD-compatible. Otherwise it looks good.

[mcabbott]

OK, done.

[MikeInnes]

Perfect, thanks a lot!