[Feat] Improve TPU CI

[tchaton]

What does this PR do?

This PR modify the TPU CI to rely on docker hub PyTorch Lightning xla-image,
saving around 4 min, bringing CI time from 12-13 min to 8-9min

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Did you have fun?

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[codecov]

Codecov Report

Merging #6078 (c596123) into master (257fabd) will increase coverage by 0%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #6078    +/-   ##
=======================================
  Coverage      92%     92%            
=======================================
  Files         217     217            
  Lines       14097   14226   +129     
=======================================
+ Hits        13004   13131   +127     
- Misses       1093    1095     +2     

[tchaton]

LGTM!