Enhance X-VLA finetuning documentation with optimizer details (#2537)

Added detailed instructions for implementing a custom optimizer and modifying parameter retrieval for X-VLA finetuning.

Signed-off-by: Jinliang Zheng <54488861+2toinf@users.noreply.github.com>

docs/source/xvla.mdx

@@ -157,6 +157,76 @@ lerobot-train \
   --policy.train_soft_prompts=True
 ```
 
+
+
+💡 **Best Performance:** If you have sufficient computational resources and want to achieve best X-VLA finetuning performance, you should follow the official finetuning strategy:
+
+**🔥 Full-finetune all components with a custom learning-rate scheme**
+
+To ensure stable optimization, the Vision-Language Model (VLM) must be trained with only 1/10 of the base learning rate, while all other components use the full LR.
+This LR ratio is crucial for achieving strong and stable finetuning performance.
+To enable this behavior, you must:
+1. Implement a custom optimizer and register it in your training config
+```
+from dataclasses import dataclass, asdict
+from lerobot.optim.optimizers import OptimizerConfig
+import torch
+
+@OptimizerConfig.register_subclass("xvla-adamw")
+@dataclass
+class XVLAAdamW(OptimizerConfig):
+    lr: float = 1e-4
+    betas: tuple[float, float] = (0.9, 0.99)
+    eps: float = 1e-8
+    weight_decay: float = 0.0
+    grad_clip_norm: float = 10.0
+
+    def build(self, params: dict) -> torch.optim.Optimizer:
+        """
+        Expect `named_parameters()` as input.
+        Apply lr = lr / 10 for all VLM-related parameters.
+        """
+        assert isinstance(params, dict), \
+            "Custom LR optimizer requires `named_parameters()` as inputs."
+        kwargs = asdict(self)
+        kwargs.pop("grad_clip_norm")
+        vlm_group, other_group = [], []
+        for name, p in params.items():
+            if not p.requires_grad:
+                continue
+            if "vlm" in name.lower():
+                vlm_group.append(p)
+            else:
+                other_group.append(p)
+
+        param_groups = [
+            {"params": vlm_group, "lr": self.lr * 0.1, "weight_decay": self.weight_decay * 0.1},
+            {"params": other_group, "lr": self.lr, "weight_decay": self.weight_decay},
+        ]
+
+        return torch.optim.AdamW(param_groups, **kwargs)
+```
+2. Modify X-VLA’s get_optim_params to return named parameters
+
+Replace:
+```
+def get_optim_params(self) -> dict:
+    """Return only trainable parameters for optimization."""
+    return filter(lambda p: p.requires_grad, self.parameters())
+```
+with:
+```
+def get_optim_params(self):
+    """Return trainable named parameters."""
+    return filter(lambda kv: kv[1].requires_grad, self.named_parameters())
+```
+This ensures the optimizer receives a dict of named parameters, allowing it to correctly detect VLM modules and apply the 1/10 LR rule.
+
+❕Note
+
+Completely matching the official reported performance may require an additional warm-up LR schedule for soft-prompts, which can bring minor improvements.
+We encourage implementing this in your customized training pipeline for optimal results.
+
 ---
 
 ## Core Concepts