Fix _merged_adapters bookkeeping desync on partial unfuse_lora(components=...)

[ErenAta16]

Fixes #14214.

What does this PR do?

LoraBaseMixin._merged_adapters (backing num_fused_loras/fused_loras) is a single set shared across the whole pipeline, but merge state itself is tracked per component by PEFT. fuse_lora()/unfuse_lora() both accept a components argument to operate on a subset of _lora_loadable_modules. When the same adapter is fused into multiple components (the default when fuse_lora() is called with no adapter_names) and later only some of those components are unfused, unfuse_lora() dropped the adapter name from _merged_adapters unconditionally, even though it's still physically merged into the base weights of the untouched component(s).

pipe.fuse_lora(components=["unet", "text_encoder"], adapter_names=["my_adapter"])
pipe.num_fused_loras  # 1

pipe.unfuse_lora(components=["text_encoder"])  # only unfuse text_encoder
pipe.num_fused_loras  # 0, but unet still has my_adapter merged into its weights!

Fix

Track which adapters got unmerged during the per-component loop, then only drop an adapter from the pipeline-wide _merged_adapters set once it's confirmed unmerged in every _lora_loadable_modules component (new _is_adapter_merged_in_any_component helper), not just the ones passed to this particular call.

Testing

Added test_fuse_unfuse_partial_components_keeps_merged_adapter_bookkeeping to the shared PeftLoraLoaderMixinTests mixin in tests/lora/utils.py, so it runs across the pipeline-specific test files that inherit from it (skips pipelines without a text_encoder loadable module, since the scenario needs at least two independent components). Ran it against StableDiffusionLoRATests:

tests/lora/test_lora_layers_sd.py::StableDiffusionLoRATests::test_fuse_unfuse_partial_components_keeps_merged_adapter_bookkeeping PASSED

Also ran the full existing fuse/unfuse suite for the same pipeline to check for regressions in the common (full-components) case:

test_lora_fuse_nan XPASS
test_simple_inference_with_text_denoiser_lora_unfused PASSED
test_simple_inference_with_text_lora_denoiser_fused PASSED
test_simple_inference_with_text_lora_denoiser_fused_multi PASSED
test_simple_inference_with_text_lora_fused PASSED

5 passed, 1 skipped (torch_compile variant, unrelated), 1 xpassed (pre-existing marker, unrelated), no regressions.

I verified the underlying bug and the fix directly with the actual LoraBaseMixin/PeftAdapterMixin classes and a real peft.LoraConfig before writing the pipeline-level test, in case that's useful:

pipe.fuse_lora(components=["unet", "text_encoder"], adapter_names=["my_adapter"])
print(pipe.num_fused_loras)  # 1

pipe.unfuse_lora(components=["text_encoder"])
print(pipe.num_fused_loras)  # before fix: 0 (wrong); after fix: 1 (correct, unet still merged)