Fix local rank zero casting

[ananthsub]

What does this PR do?

The environment variable 'LOCAL_RANK' can be a string, causing the if rank_zero_only.rank == 0 check to fail. This is similar to the casting added in #1811

Fixes #2641

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[codecov]

Codecov Report

Merging #2640 into master will decrease coverage by 0%.
The diff coverage is 50%.

@@          Coverage Diff           @@
##           master   #2640   +/-   ##
======================================
- Coverage      91%     91%   -0%     
======================================
  Files          70      70           
  Lines        5778    5775    -3     
======================================
- Hits         5270    5267    -3     
  Misses        508     508