fix GlobalPooling with graph only input

[CarloLucibello]

No description provided.

[codecov]

Codecov Report

Merging #52 (b720b05) into master (afad470) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #52      +/-   ##
==========================================
+ Coverage   79.47%   79.50%   +0.02%     
==========================================
  Files           8        8              
  Lines         726      727       +1     
==========================================
+ Hits          577      578       +1     
  Misses        149      149              

Impacted Files	Coverage Δ
src/utils.jl	81.91% <ø> (ø)
src/layers/pool.jl	100.00% <100.00%> (ø)

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