Add support for more backbones(mobilnet, vgg, densenet, resnext) & refactor

[kaushikb11]

What does this PR do?

Add support for more backbones & refactor Image Classification

Fixes # (issue)

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[codecov]

Codecov Report

Merging #45 (e780ad2) into master (58d3a84) will decrease coverage by 0.48%.
The diff coverage is 61.53%.

@@            Coverage Diff             @@
##           master      #45      +/-   ##
==========================================
- Coverage   80.98%   80.50%   -0.49%     
==========================================
  Files          46       47       +1     
  Lines        1383     1400      +17     
==========================================
+ Hits         1120     1127       +7     
- Misses        263      273      +10     

Flag	Coverage Δ
unittests	80.50% <61.53%> (-0.49%)	⬇️

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Impacted Files	Coverage Δ
flash/vision/classification/backbones.py	58.33% <58.33%> (ø)
flash/vision/classification/model.py	100.00% <100.00%> (ø)

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[teddykoker]

LGTM, just one question