[train][step-wise] Three correctness/efficiency fixes for step-wise training#1539

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CharlieFRuan wants to merge 3 commits intomainfrom

charlie/stepwise-training-opts

Commits on Apr 20, 2026

[train][step-wise] Run length check after re-tokenizing chat history
CharlieFRuan
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[train] Mask post-hoc appended EOS out of the loss
CharlieFRuan
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[train][step-wise] Drop obs_ids from per-step response
CharlieFRuan
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