New step "TRL trainer + Config auto-discovery sweep" mirrors the
auto-detection in unsloth/models/rl.py:
- rl.py:1934-1949 (`patch_trl_rl_trainers`) walks dir(trl.trainer),
keeps lowercase `<x>_trainer` names except `base_trainer`.
- rl.py:553-569 picks the unique `<prefix>*Trainer` and
`<prefix>*Config` per trainer module.
- rl.py:575-615 falls back to a sibling `<x>_config.py` module
(TRL 0.26+ split) and then to an MRO walk into experimental
parent modules (thin-wrapper trainers).
Three pytest cases per cell:
1. AST-parse every *_trainer and *_config source file on disk via
importlib.util.find_spec(...).origin. Reads files WITHOUT
triggering optional-dep imports (grpo_trainer requires vllm,
nash_md/online_dpo/rloo/xpo do too). Catches TRL source-level
drift on any matrix cell.
2. Drive unsloth's discovery rules over every trainer file.
Records ok / import-skipped / discovery-skipped / fail.
Hard-fails when a trainer imports cleanly + has 1 *Trainer but
no *Config can be resolved via the three rules.
Asserts >=3 trainers fully discover (sft/reward/dpo are the
historical core; below that signals a TRL refactor regression).
3. Orphan check: every *_trainer module must have a sibling
*_config.py OR an inline *Config; raises if neither exists,
because that combination silently breaks `_patch_trl_rl_trainers`.
Local verification on TRL 0.25.1: 31/31 modules AST-parse,
10 trainers fully discover (bco/cpo/dpo/gkd/kto/orpo/ppo/prm/reward/
sft), 5 import-skipped (grpo/nash_md/online_dpo/rloo/xpo, all need
vllm which is intentionally not installed in the CI matrix).
Wall-time ~10-30s per cell, dominated by lazy-module dir()
materialisation.
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