DiffusionLoraConfig has carried cond_cache_dir for a while and the DiT trainer
acts on it, but DiffusionTrainingStartRequest omitted the field, so Pydantic
dropped it silently and every API-driven run fell back to the in-memory cache
that is rebuilt from scratch each time. The warm path skips loading the VAE and
the multi-GB text encoders on a rerun whose images, captions and resolution are
unchanged, so this was a real capability that could not be reached.
Contained like output_dir rather than left to the trainer subprocess's cwd,
since it is another directory the trainer writes to. Blank or omitted still
means the in-memory cache, so it must not resolve to the outputs root.
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