drop install FairScale for TPU

[Borda]

What does this PR do?

seem to be quite time-consuming since it is really not needed to install fairscale on TPU...

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[codecov]

Codecov Report

Merging #5113 (7497b33) into master (3910b28) will decrease coverage by 0%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #5113   +/-   ##
======================================
- Coverage      93%     93%   -0%     
======================================
  Files         134     134           
  Lines        9928    9928           
======================================
- Hits         9242    9235    -7     
- Misses        686     693    +7     

[tchaton]

LGTM !