autograd: user custom function

[iblislin]

• an example included: examples/autograd/customfunc.jl

@vchuravy Could you review the GC-related part of this PR?

TODO:

• support where syntax for @custom

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@0fba766). Click here to learn what that means.
The diff coverage is 91.52%.

@@            Coverage Diff            @@
##             master     #385   +/-   ##
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  Coverage          ?   71.28%           
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  Files             ?       28           
  Lines             ?     2114           
  Branches          ?        0           
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  Hits              ?     1507           
  Misses            ?      607           
  Partials          ?        0

Impacted Files	Coverage Δ
src/MXNet.jl	100% <ø> (ø)
src/base.jl	36.55% <100%> (ø)
src/autograd.jl	95.96% <91.37%> (ø)

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[iblislin]

ready for review

[jekbradbury]

Any idea what's going wrong with GitHub's triple quote detection such that it thinks the whole autograd.jl is quoted?
In any case this is pretty cool -- something I'd assumed was impossible, and something (fast custom native functions in the frontend's language!) that the Python frontend can never, ever offer.

[iblislin]

Any idea what's going wrong with GitHub's triple quote detection such that it thinks the whole autograd.jl is quoted?

It was an issue of JuliaEditorSupport/atom-language-julia, and fixed in latest release. Just waiting github upgrade their dependency.
See: github-linguist/linguist#3950