Don't promote vectors to matrices in dense layers

[chriselrod]

Closes #89

[codecov]

Codecov Report

Merging #92 (4d2a78c) into main (3cda9f3) will increase coverage by 0.82%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##             main      #92      +/-   ##
==========================================
+ Coverage   71.41%   72.23%   +0.82%     
==========================================
  Files          13       13              
  Lines        2316     2316              
==========================================
+ Hits         1654     1673      +19     
+ Misses        662      643      -19     

Impacted Files	Coverage Δ
src/simple_chain.jl	87.09% <33.33%> (ø)
src/dense.jl	87.35% <100.00%> (+5.33%)	⬆️
src/forwarddiff_matmul.jl	88.68% <100.00%> (ø)

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