* fix(config): add lora_alpha to PeftConfig
PeftConfig was missing the lora_alpha field, causing the PEFT library
to default to alpha=8 regardless of the LoRA rank, which dampens the
adaptation signal for high-rank adapters (e.g., r=128).
This adds lora_alpha: int | None = None to PeftConfig, allowing users
to specify --peft.lora_alpha <value> on the CLI.
Closes#3551
* fix(docs): add lora_alpha to peft training example + clarify scaling formula
- Add --peft.lora_alpha=64 to docs/source/peft_training.mdx example to
prevent new users from hitting the alpha=8 default dampening bug
- Clarify lora_alpha comment in default.py with scaling = lora_alpha / r
* docs: mention both --peft.r and --peft.lora_alpha in LoRA description
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Co-authored-by: Cheng Yin <yin@users.noreply.github.com>
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