Add usd_decoder to mujoco/plugin. - Previously was src/experimental/usd/usd_to_mjspec.cc, now that same spec is returned by the plugin. PiperOrigin-RevId: 854350962 Change-Id: Ia980190a557c50ce8197980922a2594b613859b4
PiperOrigin-RevId: 775269423 Change-Id: Ifca6c3ead32b9fb3f02784ed38a9efae300be410
PiperOrigin-RevId: 759124593 Change-Id: Ic2fa2a00f8384f432ee2eef95c80f5c180aa4ca9