better handling of chain with only graph input

[CarloLucibello]

fix #49

[codecov]

Codecov Report

Merging #50 (6b9c3f5) into master (867cc10) will increase coverage by 0.40%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #50      +/-   ##
==========================================
+ Coverage   79.62%   80.02%   +0.40%     
==========================================
  Files           8        8              
  Lines         697      701       +4     
==========================================
+ Hits          555      561       +6     
+ Misses        142      140       -2     

Impacted Files	Coverage Δ
src/layers/basic.jl	55.55% <100.00%> (+16.42%)	⬆️

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