[docs] Update FairScale install instructions

[SeanNaren]

What does this PR do?

Fixes #5210 with latest fairscale release.

Upstream in FairScale we've added a unit test to make sure this is covered, so it might be alright for now to keep the version the same here for our unit testing. Can address in a separate PR if needed in the future.

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[carmocca]

so it might be alright for now to keep the version the same here for our unit testing

I agree with keeping that test upstream

[Borda]

Shall we update it also in requirements?

[SeanNaren]

I made a note of it in the main post. We can but it isn't necessary, the unit test for the additional fixes is in Fairscale. Till, pipe RPC becomes a bit more stable let's keep this pointed to our own fork!

[codecov]

Codecov Report

Merging #5380 (10fbc6b) into master (019e4ff) will decrease coverage by 0%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #5380   +/-   ##
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- Coverage      93%     93%   -0%     
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  Files         134     134           
  Lines        9994    9994           
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- Hits         9310    9309    -1     
- Misses        684     685    +1