Add FastSiam model

[philippmwirth]

Add FastSiam model

Closes #972.

• Add SimSiam and FastSiam transforms
• Add FastSiam model to benchmarks

I'll add the examples in a follow-up pull request.

TODO:

• Imagenette @ 200 epochs
• Imagenette @ 800 epochs
• Cifar10 benchmarks
• Imagenet100 benchmarks

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.19 🎉

Comparison is base (b7b6748) 89.35% compared to head (3dccb1a) 89.54%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1130      +/-   ##
==========================================
+ Coverage   89.35%   89.54%   +0.19%     
==========================================
  Files         129      131       +2     
  Lines        5475     5607     +132     
==========================================
+ Hits         4892     5021     +129     
- Misses        583      586       +3     

Impacted Files	Coverage Δ
lightly/transforms/__init__.py	100.00% <100.00%> (ø)
lightly/transforms/fast_siam_transform.py	100.00% <100.00%> (ø)
lightly/transforms/simsiam_transform.py	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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[guarin]

Oh, that is shorter than expected! Left a few comments :)

[guarin]

LGTM! Left two small comments but feel free to resolve and merge.