pymjcf: assets dict is too big

I decomposed my model into many pieces of convexhall meshes (about 2k), then tried to load it into pymjcf, with API `mjcf.Physics.from_mjcf_model`, it raised a `ValueError: assets dict is too big`. whether load them with .xml or python's for loop will trigger that Error. But load them with `mujoco.Physics.from_xml_string` goes all right. I don't know if this is a bug in pymjcf or it has some way to avoid. I really loves pymjcf to build my envirment.
[IssuesExample.zip](https://github.com/deepmind/dm_control/files/10979961/IssuesExample.zip)