Add group support to mjcPhysics equivalents for sites, geoms, actuators, and joints.

PiperOrigin-RevId: 781945497
Change-Id: I5c409f05e6bae33c181a51c273f260daea6004fc

src/experimental/usd/plugins/mjcf/mujoco_to_usd.cc

@@ -1463,6 +1463,9 @@ class ModelWriter {
 
     ApplyApiSchema(data_, site_path, MjcPhysicsTokens->MjcSiteAPI);
 
+    WriteUniformAttribute(site_path, pxr::SdfValueTypeNames->Int,
+                          MjcPhysicsTokens->mjcGroup, site->group);
+
     int site_id = mjs_getId(site->element);
     auto transform = MujocoPosQuatToTransform(&model_->site_pos[3 * site_id],
                                               &model_->site_quat[4 * site_id]);
@@ -1508,6 +1511,9 @@ class ModelWriter {
         return;
     }
 
+    WriteUniformAttribute(geom_path, pxr::SdfValueTypeNames->Int,
+                          MjcPhysicsTokens->mjcGroup, geom->group);
+
     // Apply the physics schemas if we are writing physics and the
     // geom participates in collisions.
     if (write_physics_ && (model_->geom_contype[geom_id] != 0 ||
@@ -1855,6 +1861,9 @@ class ModelWriter {
       // Finally write the mjcPhysicsJointAPI attributes.
       ApplyApiSchema(data_, joint_path, MjcPhysicsTokens->MjcJointAPI);
 
+      WriteUniformAttribute(joint_path, pxr::SdfValueTypeNames->Int,
+                            MjcPhysicsTokens->mjcGroup, joint->group);
+
       WriteUniformAttribute(
           joint_path, pxr::SdfValueTypeNames->DoubleArray,
           MjcPhysicsTokens->mjcSpringdamper,