Existence check for hparams now uses underlying filesystem

[kandluis]

What does this PR do?

Fixes #5249

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[codecov]

Codecov Report

Merging #5250 (241f71e) into master (d568533) will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #5250   +/-   ##
======================================
  Coverage      93%     93%           
======================================
  Files         134     134           
  Lines        9976    9976           
======================================
  Hits         9294    9294           
  Misses        682     682           

[Borda]

@awaelchli pls check is it 1.2 or 1.1.x? the description says 1.2 but the target is master…