make generate images function work with parallel in CIFAR files

[kilianFatras]

What does this PR do?

1. Remove unused FLAG variables in cifar experiments.
2. Make generate_images function work with parallel

Fixes #82 and partially 73

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (3c0cfdd) 35.90% compared to head (d964396) 35.90%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #83   +/-   ##
=======================================
  Coverage   35.90%   35.90%           
=======================================
  Files          67       67           
  Lines        7419     7419           
=======================================
  Hits         2664     2664           
  Misses       4755     4755           

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[kilianFatras]

-- DO NOT MERGE --

After two tests, the above fix does not seem to perform as well as the main branch. I got an FID of 4.9 for the two tests while it should be 3.6. For now, I consider the parallel training not fixed.