Fix transformers_trainer save_model logic

[Fridah-nv]

Change in #546 breaks some conditions of saving models in hf training example, this PR fix the issue by reverting change in checkpoint saving logic and update the config separately.

Related pipeline failure: https://gitlab-master.nvidia.com/omniml/modelopt/-/jobs/233869405

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.43%. Comparing base (bc52b6c) to head (7e2a78e).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #580      +/-   ##
==========================================
- Coverage   74.44%   74.43%   -0.02%     
==========================================
  Files         182      182              
  Lines       18234    18234              
==========================================
- Hits        13575    13572       -3     
- Misses       4659     4662       +3     

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