Add CLIP backbones for text / image classification

[ethanwharris]

What does this PR do?

• Add CLIP backbone for image and text classification
• Refactor text classifier to use adapters

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[codecov]

Codecov Report

Base: 92.29% // Head: 92.33% // Increases project coverage by +0.04% 🎉

Coverage data is based on head (5e36793) compared to base (dc30097).
Patch coverage: 95.93% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1458      +/-   ##
==========================================
+ Coverage   92.29%   92.33%   +0.04%     
==========================================
  Files         287      291       +4     
  Lines       13061    13197     +136     
==========================================
+ Hits        12054    12185     +131     
- Misses       1007     1012       +5     

Flag	Coverage Δ
pytest	13.16% <0.58%> (-0.13%)	⬇️
tpu	13.16% <0.58%> (-0.13%)	⬇️
unittests	92.86% <95.93%> (+0.03%)	⬆️

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

Impacted Files	Coverage Δ
flash/image/classification/adapters.py	82.53% <ø> (ø)
flash/image/segmentation/model.py	94.31% <0.00%> (ø)
flash/core/utilities/embedder.py	88.88% <85.71%> (-0.70%)	⬇️
flash/text/classification/adapters.py	94.94% <94.94%> (ø)
flash/core/registry.py	96.20% <100.00%> (+0.07%)	⬆️
flash/core/utilities/providers.py	100.00% <100.00%> (ø)
flash/image/classification/backbones/__init__.py	100.00% <100.00%> (ø)
flash/image/classification/backbones/clip.py	100.00% <100.00%> (ø)
flash/text/classification/backbones/__init__.py	100.00% <100.00%> (ø)
flash/text/classification/backbones/clip.py	100.00% <100.00%> (ø)
... and 3 more

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[kaushikb11]

Thanks Ethan!