chore: sync lora implementation with upstream

[emergenz]

We are currently using our own LoRA implementation, while miles will rely on PEFT as soon as their implementation is merged into main. Thus we should migrate to PEFT. Also, we should match the code structure of the two PRs in order to avoid merge conflicts as much as possible down the line.

Miles LoRA PRs:

radixark#377

radixark#326

Changes

Aspect	Old	New (PR 326 compatible)
Enable LoRA	--use-lora flag	--lora-rank > 0
Target modules	--lora-target-modules	--target-modules
LoRA detection	args.use_lora	is_lora_model(model)
Checkpoint dir	checkpoint_dir/	checkpoint_dir/adapter/
State dict key	"model"	"adapter" (when LoRA)
LoRA library	Custom LoRALinear	PEFT library