Add Ideogram4LoraLoaderMixin (LoRA loading for Ideogram4)

[linoytsaban]

Adds Ideogram4LoraLoaderMixin and wires it into Ideogram4Pipeline, so Ideogram 4 pipelines can load_lora_weights / save_lora_weights / fuse like the other models.

This is the loading foundation split out of the Ideogram4 work so it can be reviewed/merged on its own. Two follow-ups are stacked on top of this branch:

• Ideogram4 lora training #13861 — the DreamBooth LoRA training script (rebased onto this).
• Support loading non-diffusers Ideogram4 LoRA checkpoints #13919 — loading non-diffusers (ai-toolkit/ostris, fal) Ideogram4 LoRAs (rebased onto this).

Both depend on this mixin; this PR is independent of the training script's readiness.

[HuggingFaceDocBuilderDev]

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