[peft] Given that self.active_adapter is deprecated, avoid using it

[tomaarsen]

What does this PR do?

PeftAdapterMixin.active_adapter has been deprecated since #26407 added multi-adapter support, but it's still being used in PeftAdapterMixin.get_adapter_state_dict. This results in a FutureWarning every time I save a checkpoint after training with model.add_adapter(...).

This PR updates the latter to use the correct PeftAdapterMixin.active_adapters()[0] method instead, which is identical in behaviour, except minus the warning.

The deprecated active_adapter:

transformers/src/transformers/integrations/peft.py

Lines 433 to 438 in 54739a3

	def active_adapter(self) -> str:
	warnings.warn(
	"The `active_adapter` method is deprecated and will be removed in a future version.", FutureWarning
	)
	return self.active_adapters()[0]

Additionally, the comment

The model will use `self.active_adapter()`

in enable_adapters seemed to be a bit misleading - that method wasn't being used. For clarity, I removed it.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

@ArthurZucker @BenjaminBossan

• Tom Aarsen

[HuggingFaceDocBuilderDev]

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[BenjaminBossan]

Thanks for fixing this, LGTM.

[ArthurZucker]

Thanks!