feat(pi0): add train_expert_only and freeze_vision_encoder flags to pi0 and pi0.5 (#2727)

* feat(pi0): add train_expert_only and freeze_vision_encoder options

* pi_05: train_expert_only and freeze_vision_encoder flags

* comment clean up

* docs: add finetuning parameters to pi0 and pi05 docs

* updating docs to follow standards

docs/source/pi0.mdx

@@ -64,6 +64,8 @@ python src/lerobot/scripts/lerobot_train.py \
     --policy.compile_model=true \
     --policy.gradient_checkpointing=true \
     --policy.dtype=bfloat16 \
+    --policy.freeze_vision_encoder=false \
+    --policy.train_expert_only=false \
     --steps=3000 \
     --policy.device=cuda \
     --batch_size=32
@@ -79,6 +81,15 @@ python src/lerobot/scripts/lerobot_train.py \
   - [lerobot/pi0_base](https://huggingface.co/lerobot/pi0_base)
   - [lerobot/pi0_libero](https://huggingface.co/lerobot/pi0_libero) (specifically trained on the Libero dataset)
 
+### Training Parameters Explained
+
+| Parameter               | Default | Description                                 |
+| ----------------------- | ------- | ------------------------------------------- |
+| `freeze_vision_encoder` | `false` | Do not freeze the vision encoder            |
+| `train_expert_only`     | `false` | Do not freeze the VLM, train all parameters |
+
+**💡 Tip**: Setting `train_expert_only=true` freezes the VLM and trains only the action expert and projections, allowing finetuning with reduced memory usage.
+
 ## License
 
 This model follows the **Apache 2.0 License**, consistent with the original [OpenPI repository](https://github.com/Physical-Intelligence/openpi).