Attachment frames's inertial property

Hello,

I am trying to attach a body to another one, similar to your example [here](https://github.com/google-deepmind/dm_control/blob/ed424509c0a0e8ddf7a43824924de483026ad9cc/dm_control/mjcf/README.md#attachment-frames) of `child` and `parent`.

However, my `child` body does have inertial-properties. Attaching the following `child` body
```
class Camera:
#===================#
# C O N S T A N T #
#===================#
_com_pos = (0, 0, 0.015)
_dclass = 'wam\\viz'
_mass = 0.095
_mesh_name = 'intel_realsense_l515'
_name = '\\rgb'
_quat = (0, 0, 1, 0)
#===============================#
# I N I T I A L I Z A T I O N #
#===============================#
def __init__(self, name, fovy):
# Make a MujoCo Root
self.mjcf_model = mjcf.RootElement()
# Add a body element: No need to add position info, as that will be added based on which `site` this body is going to be attached (https://github.com/google-deepmind/dm_control/blob/main/dm_control/mjcf/README.md#attachment-frames)
self.camera_body = self.mjcf_model.worldbody.add('body',
name = name)
# Add Inertial properties
self.camera_body.add('inertial',
pos = self._com_pos,
mass = self._mass)
# Add the Geometry (https://github.com/google-deepmind/dm_control/blob/ed424509c0a0e8ddf7a43824924de483026ad9cc/dm_control/locomotion/soccer/humanoid.py#L50)
self.camera_body.add('geom',
mesh = self._mesh_name,
dclass = self._dclass)
# Add camera sensor
self.camera_body.add('camera',
name = name+self._name,
pos = (0, 0, 0),
quat = self._quat,
fovy = fovy)
```
as such in the `parent` body
```
# Add camera locations
# An empty array to hold camera-sites
camera_site = []
for x in range(len(self._camera_pos)):
camera_site.append(self.summit_body.add('site',
pos = self._camera_pos[x],
quat = self._camera_quat[x],
dclass = self._site_name))
# Create camera
# Initialize empty list of camera
cameras = []
for x in range(len(self._camera_pos)):
cameras.append(Camera(name = self._name_summit+'_'+prefix+self._camera_name[x],
fovy = self._camera_fovy[x]))
# Attach the cameras
for x in range(len(self._camera_pos)):
camera_site[x].attach(cameras[x].mjcf_model)
```
causes the generated file to introduce an unnecessary body called `unnamed_model`, which is derived from the name of the `worldbody` (which, in this case, was initialized without a name).
```

```
What I am aiming for is a representation similar to your example [here](https://github.com/google-deepmind/dm_control/blob/ed424509c0a0e8ddf7a43824924de483026ad9cc/dm_control/mjcf/README.md#attachment-frames), such that I should have:
```

```
but I am not allowed to do this because `AttributeError: is not a valid child of `.

Question;
How can I get such a clean representation while introducing `inertial` properties for attached-bodies?