Make checkpoint saving folder clear in the config

[DNXie]

Made it clear in the config where to specify the checkpoint saving folder. And added some necessary comments to clarify the behavior.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.63%. Comparing base (633b219) to head (8b9d30c).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #444      +/-   ##
==========================================
- Coverage   64.69%   64.63%   -0.06%     
==========================================
  Files          79       79              
  Lines        7775     7788      +13     
==========================================
+ Hits         5030     5034       +4     
- Misses       2745     2754       +9     

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[JenniferWang]

I think we control these config fields and we should be opinionated on exposing RL friendly config field names and re-map it to TorchTitan fields internally.

Right now, some TorchTitan config names are really confusing: e.g. the ref model logically should not need checkpointing but still it requires checkpoint.enable = true.

[ebsmothers]

Yeah was discussing this with @joecummings too.. ultimately we probably want to provide some kind of internal config mapping that we execute as a kind of training script post-init before we do any of the actual setup of actors (e.g. we can even bake it into our config.parse decorator).