ArticulationView get_link_velocities detaches gradients

[pschoc]

I have set up a global model:
self.model = self.builder.finalize(requires_grad=False)

then I have created an ArticulationView object to conveniently reference different joints of this robot:
self.robot = ArticulationView(self.model, "fr3", verbose=True, exclude_joint_types=[newton.JOINT_FREE])

Now say I have a state reference with grads enabled:
temp_state_for_jacobian = self.model.state(requires_grad=True)

(checking via debugger that gradients are really attached: temp_state_for_jacobian.body_qd.requires_grad = True)

however, when trying to access body_qd via the ArticulationView interface, my gradients get detached:
self.robot.get_attribute("body_qd", temp_state_for_jacobian)[0].requires_grad = False
or
self.robot.get_link_velocities(temp_state_for_jacobian).requires_grad = False

this is not the case for joint positions and velocities, therefore I checked the getter inside selection.py and saw that the staging_array access is preferred if available

original code:

def _get_attribute_values(self, name: str, source: Model | State | Control, _slice: slice | None = None):
attrib = self._get_attribute_array(name, source, _slice=_slice)
if hasattr(attrib, "_staging_array"):
wp.copy(attrib._staging_array, attrib)
return attrib._staging_array
else:
return attrib

proposing to check if the original tensor requires grad:
def _get_attribute_values(self, name: str, source: Model | State | Control, _slice: slice | None = None):
attrib = self._get_attribute_array(name, source, _slice=_slice)
if hasattr(attrib, "data") and attrib.data.requires_grad:
return attrib.data
else:
if hasattr(attrib, "_staging_array"):
wp.copy(attrib._staging_array, attrib)
return attrib._staging_array
else:
return attrib

I dont fully understand what the correct fix would be but this worked for my case