[bezabyte]

Fixes #50677

When Trainer resumes from an older checkpoint in the same output_dir while newer checkpoints exist, log a warning explaining that later saves can overwrite those checkpoints and that resume_from_checkpoint=True may select a newer checkpoint after an interruption.

This also documents using a new output_dir for rollback and adds a regression test.

Validation:

• Direct rollback-warning smoke test passed.
• python -m compileall -q src/transformers/trainer.py tests/trainer/test_trainer_checkpointing.py passed.
• git diff --check passed.
• Full Trainer test could not run locally because accelerate is not installed; pytest collection also hits the environment's missing xdist hook.

[github-actions]

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[bezabyte]

Before / after comparison:

• Before: resuming from checkpoint-2 while newer checkpoints existed emitted no warning.
• After: Trainer warns about the newer checkpoints and recommends a new output_dir for rollback.
  These screenshots come from the rollback-warning smoke test.