Two handy little things landed in the trainer this time~

• 🧊 Caching: VAE latents and text-encoder outputs can now be pre-computed and stored ahead of time! Keep them in RAM or on disk. No more recomputing every step — less VRAM, faster training, yay~
• 🎯 UNet-only training: A new train_text_encoder switch (off by
  default). When off, only the UNet gets LoRA and the text encoders get to rest, so training stays light and breezy!
• 📝 Also refreshed the config example and added plenty of tests so nothing breaks ✅

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