try PT 1.8 compatible fairscale

[Borda]

What does this PR do?

Fixing compatibility with fairscale and PT 1.8
cc: @SeanNaren

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[codecov]

Codecov Report

Merging #5996 (7dfdf58) into master (fcfa7fa) will decrease coverage by 59%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #5996     +/-   ##
========================================
- Coverage      90%     31%    -59%     
========================================
  Files         170     170             
  Lines       11663   11663             
========================================
- Hits        10533    3661   -6872     
- Misses       1130    8002   +6872     

[tchaton]

LGTM !