Fix mutable default args in lora_base.py

[PrakshaaleJain]

What does this PR do?

Fixes #14052

This PR fixes a bug in src/diffusers/loaders/lora_base.py where a mutable default argument (components: list[str] = []) was used in the signatures of fuse_lora and unfuse_lora. It updates the default value to None and initializes it as a clean, empty list inside the method body if no explicit argument is passed.

Why this change is important:

1. Prevents Side Effects & State Leakage:
2. Robustness in Workflows:

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Who can review?

Core library / PEFT integrations:

• @sayakpaul @yiyixuxu @DN6 @BenjaminBossan @dg845

[sayakpaul]

Thanks! Do we have an example reproducer reflecting the issue that this PR attempts to solve?

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[PrakshaaleJain]

Hi @sayakpaul!

Although fuse_lora() raises early when components=[], the mutable default argument is still a latent Python bug. Since default arguments are shared across calls, mutating the default list changes the function's default value for the whole Python process.

We can reproduce this directly on the live LoraBaseMixin code using standard reflection without spinning up any pipelines:

import inspect
from diffusers.loaders.lora_base import LoraBaseMixin

sig = inspect.signature(LoraBaseMixin.fuse_lora)
default_object = sig.parameters["components"].default

print(f"Initial default type: {type(default_object)}")  # Expected: <class 'list'>
default_object.append("leaked_state_or_component")

new_sig = inspect.signature(LoraBaseMixin.fuse_lora)
print(f"Mutated default value: {new_sig.parameters['components'].default}") # Expected: ['leaked_state_or_component']
assert len(new_sig.parameters["components"].default) > 0, "Global signature state polluted!"

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Hi @PrakshaaleJain, 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.