[lora_conversion] Enhance key handling for OneTrainer components in LORA conversion utility (#11441)

[iamwavecut]

What does this PR do?

Fixes #11441

I tried to keep changeset somehow minimal, while doing it the same way as it's already done.

Before submitting

• Did you read the contributor guideline?
• Did you read our philosophy doc (important for complex PRs)?
• Was this discussed/approved via a GitHub issue or the forum? Unable to load Flux LoRA trained with OneTrainer – NotImplementedError in _convert_mixture_state_dict_to_diffusers #11441
• Did you make sure to update the documentation with your changes? - No changes
• Did you write any new necessary tests?

Who can review?

@sayakpaul would be the most relevant I suppose.

[sayakpaul]

I think either NotImplementedError or an elaborated error message with NotImplementedError is fine:

Suggested change

	raise NotImplementedError(k)
	raise NotImplementedError(f"Handling for key ({k}) is not implemented.")

[HuggingFaceDocBuilderDev]

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[sayakpaul]

Amazing, thanks! I just ran the integration LoRA tests for Flux and they all pass.