Move idx arithmetics in pooling outside the parallel loop

[aobolensk]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 92.59259% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.04%. Comparing base (a04122b) to head (e470fd0).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
include/layers/PoolingLayer.hpp	92.59%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #280      +/-   ##
==========================================
+ Coverage   84.59%   85.04%   +0.45%     
==========================================
  Files          59       59              
  Lines        3622     3624       +2     
  Branches     2170     2164       -6     
==========================================
+ Hits         3064     3082      +18     
  Misses        267      267              
+ Partials      291      275      -16     

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