Fix VISSL on GPU and add VISSL GPU CI

[ethanwharris]

What does this PR do?

Fixes # (issue)

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[codecov]

Codecov Report

Merging #1256 (ddec33d) into master (2a09ce0) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1256      +/-   ##
==========================================
+ Coverage   91.17%   91.19%   +0.02%     
==========================================
  Files         286      286              
  Lines       12789    12791       +2     
==========================================
+ Hits        11660    11665       +5     
+ Misses       1129     1126       -3     

Flag	Coverage Δ
unittests	91.19% <100.00%> (+0.02%)	⬆️

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

Impacted Files	Coverage Δ
flash/core/utilities/imports.py	92.85% <100.00%> (ø)
flash/image/embedding/model.py	98.03% <100.00%> (+5.88%)	⬆️
flash/image/embedding/vissl/adapter.py	92.39% <100.00%> (+0.08%)	⬆️
flash/image/embedding/vissl/hooks.py	98.24% <100.00%> (+0.03%)	⬆️

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[krshrimali]

Thanks, @ethanwharris - just a couple of comments, but overall looks great. 🔥 🚀