Add fairscale to VISSL requirements

[ethanwharris]

What does this PR do?

Fixes # (issue)

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[Borda]

how is this required?

[ethanwharris]

VISSL tests require it but somehow it isn't listed in their requirements. I think because it used to get installed with PL maybe? Actually looks liek VISSL made a new version that removes it from their reqs. for some reason...

[codecov]

Codecov Report

Merging #909 (6619bd4) into master (7618408) will decrease coverage by 3.77%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #909      +/-   ##
==========================================
- Coverage   82.34%   78.57%   -3.78%     
==========================================
  Files         237      237              
  Lines       12885    12885              
==========================================
- Hits        10610    10124     -486     
- Misses       2275     2761     +486     

Flag	Coverage Δ
unittests	78.57% <100.00%> (-3.78%)	⬇️

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

Impacted Files	Coverage Δ
flash/image/embedding/model.py	41.66% <100.00%> (ø)
flash/text/question_answering/model.py	28.27% <0.00%> (-62.76%)	⬇️
flash/text/question_answering/data.py	27.93% <0.00%> (-59.05%)	⬇️
flash/text/classification/cli.py	53.84% <0.00%> (-46.16%)	⬇️
flash/text/classification/data.py	55.49% <0.00%> (-41.89%)	⬇️
flash/text/question_answering/cli.py	60.00% <0.00%> (-40.00%)	⬇️
flash/text/seq2seq/core/utils.py	70.00% <0.00%> (-30.00%)	⬇️
flash/text/seq2seq/core/metrics.py	74.07% <0.00%> (-25.00%)	⬇️
flash/text/seq2seq/core/data.py	65.85% <0.00%> (-24.40%)	⬇️
flash/text/classification/model.py	68.51% <0.00%> (-24.08%)	⬇️
... and 7 more

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