Resolve replication templates against the active backend

The {backend} placeholder follows the user's backend choice via the
factory's existing resolution instead of a class-pinned name; an
unavailable backend fails naturally at import. This also lets
SurfaceGripperCfg use the template like every other cfg.

Author: ooctipus

source/isaaclab/isaaclab/assets/asset_base.py

@@ -20,6 +20,7 @@
 from isaaclab.physics import PhysicsEvent, PhysicsManager
 from isaaclab.sim.simulation_context import SimulationContext
 from isaaclab.sim.utils.stage import get_current_stage
+from isaaclab.utils.backend_utils import FactoryBase
 from isaaclab.utils.string import string_to_callable
 
 if TYPE_CHECKING:
@@ -121,11 +122,11 @@ def _queue_replication(self, cfg: AssetBaseCfg) -> None:
         """Queue the replication contexts directed by ``cfg``.
 
         The cfg's :attr:`~isaaclab.assets.AssetBaseCfg.replication` field is the source of
-        truth; a string reference resolves through the class's ``__backend_name__``.
+        truth; a string reference resolves against the active physics backend.
         """
         replication = cfg.replication
         if isinstance(replication, str):
-            mod_name, attr_name = replication.format(backend=self.__backend_name__).rsplit(":", 1)
+            mod_name, attr_name = replication.format(backend=FactoryBase._get_backend()).rsplit(":", 1)
             replication = getattr(importlib.import_module(mod_name), attr_name)
         if replication:
             contexts = [string_to_callable(c) if isinstance(c, str) else c for c in replication]

source/isaaclab_physx/isaaclab_physx/assets/surface_gripper/surface_gripper_cfg.py

@@ -17,8 +17,11 @@
 class SurfaceGripperCfg(AssetBaseCfg):
     """Configuration parameters for a surface gripper actuator."""
 
-    replication = ("isaaclab.cloner:UsdReplicateContext", "isaaclab_physx.cloner:PhysxReplicateContext")
-    """Replication contexts for the gripper. See :attr:`~isaaclab.assets.AssetBaseCfg.replication`."""
+    replication = "isaaclab_{backend}.cloner:REPLICATION"
+    """Replication directed by the active backend's default stack.
+
+    See :attr:`~isaaclab.assets.AssetBaseCfg.replication`.
+    """
 
     prim_path: str = MISSING
     """The expression to find the grippers in the stage."""