feat: Improved YOLO concatdownsample layer's efficiency

[frgfm]

This PR improves the concatenated downsampling operation of YOLO architectures by:

• discarding the channel order and switching to torch.cat to increase inference speed
• updating unittest
• updating default object detection argument

[codecov]

Codecov Report

Merging #48 into master will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #48      +/-   ##
==========================================
- Coverage   85.36%   85.33%   -0.03%     
==========================================
  Files          29       29              
  Lines        1524     1521       -3     
==========================================
- Hits         1301     1298       -3     
  Misses        223      223              

Impacted Files	Coverage Δ
holocron/nn/functional.py	94.79% <100.00%> (-0.16%)	⬇️
holocron/nn/modules/conv.py	100.00% <100.00%> (ø)