Enable LoRA loading on ErnieImageModularPipeline

[SamuelTallet]

What does this PR do?

This tiny PR allows using load_lora_weights and unload_lora_weights directly on ErnieImageModularPipeline.

I applied same mechanism (LoRA mixin) used in other modular pipelines (Z-Image, FLUX.2, etc).

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Who can review?

@asomoza

[github-actions]

Hi @SamuelTallet, thanks for the PR! It does not appear to link an issue it fixes. If this PR addresses an existing issue, please add a closing keyword (e.g. Fixes #1234) to the PR description so the issue is linked. See the contribution guide for more details. If this PR intentionally does not fix a tracked issue, a maintainer can add the no-issue-needed label to silence this reminder.