Add user sensor bindings.

Stop force-setting nusersensor, instead allow MuJoCo to infer the size.

PiperOrigin-RevId: 494783883
Change-Id: Ia2a1b1803b2904f2dded7b790d01eb6f1920dbaf

unity/Runtime/Components/MjScene.cs

@@ -401,12 +401,17 @@ public class MjScene : MonoBehaviour {
     MjRoot.AppendChild(GenerateMjcfSection(
         doc, components.Where(component => component is MjBaseConstraint), "equality"));
 
-    MjRoot.AppendChild(GenerateMjcfSection(
-        doc, components.Where(component => component is MjActuator), "actuator"));
-
-    MjRoot.AppendChild(GenerateMjcfSection(
-        doc, components.Where(component => component is MjBaseSensor), "sensor"));
+    MjRoot.AppendChild(
+        GenerateMjcfSection(doc,
+                            components.Where(component => component is MjActuator)
+                                .OrderBy(component => component.transform.GetSiblingIndex()),
+                            "actuator"));
 
+    MjRoot.AppendChild(
+        GenerateMjcfSection(doc,
+                            components.Where(component => component is MjBaseSensor)
+                                .OrderBy(component => component.transform.GetSiblingIndex()),
+                            "sensor"));
     // Generate the Mjcf of the runtime dependencies added to the context.
     _generationContext.GenerateMjcf(MjRoot);
     return doc;