add swav and simclr models to imageclassifier + backbone reorg

[teddykoker]

What does this PR do?

Lets you do:

ImageClassifier(backbone="swav-imagenet")
ImageClassifier(backbone="simclr-imagenet")

Also reorgs all the model backbone loading to be in the same place

[codecov]

Codecov Report

Merging #68 (9e279a5) into master (aeb0063) will increase coverage by 0.11%.
The diff coverage is 92.30%.

@@            Coverage Diff             @@
##           master      #68      +/-   ##
==========================================
+ Coverage   86.50%   86.61%   +0.11%     
==========================================
  Files          46       45       -1     
  Lines        1363     1367       +4     
==========================================
+ Hits         1179     1184       +5     
+ Misses        184      183       -1     

Flag	Coverage Δ
unittests	86.61% <92.30%> (+0.11%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
flash/vision/backbones.py	90.90% <91.42%> (-4.93%)	⬇️
flash/vision/classification/model.py	100.00% <100.00%> (ø)
flash/vision/embedding/image_embedder_model.py	80.00% <100.00%> (+3.07%)	⬆️

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