revert #9125 / fix back-compatibility with saving hparams as a whole container

[awaelchli]

This reverts commit 904dde7, added in #9125.

Fixes #9631

The tests added in #9125 still pass. The new test added here fails on master and asserting the behavior how it was before #9125

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[codecov]

Codecov Report

Merging #9642 (d590033) into master (a71be50) will decrease coverage by 0%.
The diff coverage is 85%.

@@          Coverage Diff           @@
##           master   #9642   +/-   ##
======================================
- Coverage      93%     93%   -0%     
======================================
  Files         179     179           
  Lines       15317   15305   -12     
======================================
- Hits        14209   14197   -12     
  Misses       1108    1108           

[tchaton]

LGTM !