refactor(backend/x/gorgonnx): reduce cyclomatic complexity

[rlespinasse]

With reduction of cyclomatic complexities in

• ggnBroadcastof backend/x/gorgonnx/broadcast.go
• (*maxpool).init of backend/x/gorgonnx/maxpool.go
• (*conv).init of backend/x/gorgonnx/conv.go
• (*gap).Do of backend/x/gorgonnx/gap.go
• (*gemm).do64 of backend/x/gorgonnx/gemm.go
• (*gemm).do32 of backend/x/gorgonnx/gemm.go
• (*fastBatchnorm).Do of backend/x/gorgonnx/batchnorm_op.go
• (*fastBatchnorm).check of backend/x/gorgonnx/batchnorm_op.go

Also fix a bug on wrongful broadcast when node a shape is equals to 0.

Linked to #130

[codecov[bot]]

Codecov Report

Merging #157 into master will increase coverage by 0.31%. The diff coverage is 63.2%.

@@            Coverage Diff             @@
##           master     #157      +/-   ##
==========================================
+ Coverage   86.47%   86.78%   +0.31%     
==========================================
  Files         395      395              
  Lines       10504    10525      +21     
==========================================
+ Hits         9083     9134      +51     
+ Misses       1224     1194      -30     
  Partials      197      197

Impacted Files	Coverage Δ
backend/x/gorgonnx/maxpool.go	87.76% <100%> (+2.55%)	:arrow_up:
backend/x/gorgonnx/gap.go	48.51% <32.35%> (-2.04%)	:arrow_down:
backend/x/gorgonnx/gemm.go	77.83% <46.66%> (+1.73%)	:arrow_up:
backend/x/gorgonnx/broadcast.go	61.4% <61.53%> (+26.33%)	:arrow_up:
backend/x/gorgonnx/batchnorm_op.go	70.21% <71.42%> (+1.67%)	:arrow_up:
backend/x/gorgonnx/conv.go	53.09% <90.9%> (+6.3%)	:arrow_up:

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