feat: Added StackUpsample2d

[frgfm]

This PR introduces the following modifications:

• added stack upsampling as inverse ops of concat downsampling
• added corresponding unittests
• added schema in documentation

[codecov]

Codecov Report

Merging #132 (d3ac78d) into master (de25835) will increase coverage by 0.13%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #132      +/-   ##
==========================================
+ Coverage   91.52%   91.65%   +0.13%     
==========================================
  Files          50       51       +1     
  Lines        2866     2887      +21     
==========================================
+ Hits         2623     2646      +23     
+ Misses        243      241       -2     

Impacted Files	Coverage Δ
holocron/models/segmentation/unet.py	98.20% <ø> (ø)
holocron/nn/modules/attention.py	100.00% <ø> (ø)
holocron/nn/modules/conv.py	98.70% <ø> (ø)
holocron/nn/modules/downsample.py	92.75% <ø> (ø)
holocron/nn/modules/dropblock.py	94.11% <ø> (ø)
holocron/nn/modules/lambda_layer.py	93.87% <ø> (ø)
holocron/ops/boxes.py	98.24% <ø> (ø)
holocron/nn/functional.py	92.89% <100.00%> (+0.32%)	⬆️
holocron/nn/modules/__init__.py	100.00% <100.00%> (ø)
holocron/nn/modules/upsample.py	100.00% <100.00%> (ø)
... and 2 more