diffusion_train_extras hosts the opt-in training extras: LoRAEMA shadows only
the trainable adapter params (warmup-ramped decay, default 0.99, exported as a
second adapter under output_dir/ema), PersistentConditioningCache stores latent
posterior stats and caption embeddings as safetensors keyed by content hash +
family + resolution, and the aspect-ratio bucketing helpers group mixed-aspect
datasets into same-area divisor-snapped shapes.
The DiT trainer wires the first two behind config flags that default to the
current behavior: ema_decay (0 disables) and cond_cache_dir (None disables).
A fully warm cache skips loading the VAE and text encoders entirely; a cache
hit is bit-identical to a fresh encode, including the per-channel qwen latent
normalization. Also fixes the stale _gather_sigmas call in the perf test that
still passed the scheduler instead of the sigma table.
