Fix grad

[GiggleLiu]

No description provided.

[codecov]

Codecov Report

Merging #41 (aa630fd) into master (d95816b) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #41      +/-   ##
==========================================
+ Coverage   81.11%   81.18%   +0.06%     
==========================================
  Files          25       25              
  Lines        1091     1095       +4     
==========================================
+ Hits          885      889       +4     
  Misses        206      206              

Impacted Files	Coverage Δ
src/autodiff/vars.jl	72.09% <100.00%> (+1.36%)	⬆️

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