fix(config): add lora_alpha to PeftConfig (#3573)

* 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

---------

Co-authored-by: Cheng Yin <yin@users.noreply.github.com>

docs/source/peft_training.mdx

@@ -28,13 +28,15 @@ lerobot-train \
  --steps=100000 \
  --batch_size=32 \
  --peft.method_type=LORA \
- --peft.r=64
+ --peft.r=64 \
+ --peft.lora_alpha=64
 ```
 
 Note the `--peft.method_type` parameter that let's you select which PEFT method to use. Here we use
 [LoRA](https://huggingface.co/docs/peft/main/en/package_reference/lora) (Low-Rank Adapter) which is probably the most
 popular fine-tuning method to date. Low-rank adaption means that we only fine-tune a matrix with comparably low rank
-instead of the full weight matrix. This rank can be specified using the `--peft.r` parameter. The higher the rank
+instead of the full weight matrix. This rank can be specified using the `--peft.r` parameter, and the LoRA scaling factor with
+`--peft.lora_alpha` (where `scaling = lora_alpha / r`). The higher the rank
 the closer you get to full fine-tuning
 
 There are more complex methods that have more parameters. These are not yet supported, feel free to raise an issue