fix(pi05): use fused AdamW by default

Route full PI05/PI052 fine-tuning through PyTorch's fused AdamW path to avoid the single-tensor Adam denominator allocation near GPU memory limits.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/lerobot/optim/optimizers.py

@@ -105,6 +105,7 @@ class AdamWConfig(OptimizerConfig):
     weight_decay: float = 1e-2
     grad_clip_norm: float = 10.0
     foreach: bool | None = None
+    fused: bool | None = None
 
     def build(self, params: OptimizerParams) -> torch.optim.Optimizer:
         kwargs = asdict(self)

src/lerobot/policies/pi05/configuration_pi05.py

@@ -94,6 +94,7 @@ class PI05Config(PreTrainedConfig):
     optimizer_weight_decay: float = 0.01
     optimizer_grad_clip_norm: float = 1.0
     optimizer_foreach: bool | None = False
+    optimizer_fused: bool | None = True
 
     # Scheduler settings: see openpi `CosineDecaySchedule`
     # Note: These will auto-scale if --steps < scheduler_decay_steps
@@ -154,6 +155,7 @@ class PI05Config(PreTrainedConfig):
             weight_decay=self.optimizer_weight_decay,
             grad_clip_norm=self.optimizer_grad_clip_norm,
             foreach=self.optimizer_foreach,
+            fused=self.optimizer_fused,
         )
 
     def get_scheduler_preset(self):