fix: Expose BuiltinTrainer API to users

[Electronic-Waste]

What this PR does / why we need it:

This PR exposes the BuiltinTrainer API to users, and also resolve some bugs:

1. Missing retrieving logic of ACCESS_TOKEN
2. Wrong torchtune config overrides

And the testing scripts and results can be found at: kubeflow/trainer#2675

/cc @kubeflow/wg-training-leads @astefanutti

Which issue(s) this PR fixes (optional, in Fixes #<issue number>, #<issue number>, ... format, will close the issue(s) when PR gets merged):

Fixes #

Checklist:

• Docs included if any changes are user facing

[google-oss-prow]

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