[LoRA] fix indexing in LoRA state dict expansion utils

[sayakpaul]

What does this PR do?

We introduced a bug in

diffusers/src/diffusers/loaders/lora_pipeline.py

Line 2471 in 83da817

base_weight_param = transformer_state_dict[base_param_name]

This makes the following code throw errors with the latest diffusers:

Reproducer

from diffusers import DiffusionPipeline 
import torch 

lora_one = "Purz/choose-your-own-adventure"
lora_two = "ByteDance/Hyper-SD"

pipeline = DiffusionPipeline.from_pretrained("black-forest-labs/FLUX.1-dev", torch_dtype=torch.bfloat16).to("cuda")
pipeline.load_lora_weights(lora_one) # ['default_0']
print(pipeline.get_active_adapters())

pipeline.load_lora_weights(lora_two, weight_name="Hyper-FLUX.1-dev-8steps-lora.safetensors")
print(pipeline.get_active_adapters()) # ['default_1']

pipeline.set_adapters(["default_0", "default_1"])
print(pipeline.get_active_adapters()) # ['default_0', 'default_1']

This went uncaught because we don't have a test to mimic this scenario.

So, this PR fixes this behavior and also adds a test for it.

I think we have to do a patch release for this (which I can take care of). Related to #10392.

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[sayakpaul]

Closing in favor of #10388. Cc: @hlky