Add peft as extra dependency, mark tests

Fast tests currently fail because of the missing dependency.

docs/source/peft_training.mdx

@@ -4,6 +4,8 @@
 large pretrained models such as pre-trained policies (e.g., SmolVLA, π₀, ...) to new tasks without training all
 of the model's parameters while yielding comparable performance.
 
+Install the `lerobot[peft]` optional package to enable PEFT support.
+
 To read about all the possible methods of adaption, please refer to the [🤗 PEFT docs](https://huggingface.co/docs/peft/index).
 
 ## Training SmolVLA