Modify mjcf schema to work better with MuJoCo plugins.

PiperOrigin-RevId: 500735801
Change-Id: Ie0802690104a80c42a021cbc5c7c3d0df3da25f6

Author: DeepMind

dm_control/mjcf/schema.xml

@@ -790,10 +790,10 @@
       <children>
         <element name="required" repeated="true" namespace="plugin">
           <attributes>
-            <attribute name="plugin" type="identifier" required="true"/>
+            <attribute name="plugin" type="string" required="true"/>
           </attributes>
           <children>
-            <element name="instance" repeated="true" namespace="plugin_instance">
+            <element name="instance" repeated="true" namespace="plugin">
               <attributes>
                 <attribute name="name" type="identifier" required="true"/>
               </attributes>
@@ -1762,8 +1762,8 @@
           <attributes>
             <attribute name="name" type="identifier"/>
             <attribute name="class" type="reference" reference_namespace="default"/>
-            <attribute name="plugin" type="reference" reference_namespace="plugin"/>
-            <attribute name="instance" type="reference" reference_namespace="plugin_instance"/>
+            <attribute name="plugin" type="string" reference_namespace="plugin"/>
+            <attribute name="instance" type="reference" reference_namespace="plugin"/>
             <attribute name="group" type="int"/>
             <attribute name="ctrllimited" type="keyword" valid_values="false true auto"/>
             <attribute name="forcelimited" type="keyword" valid_values="false true auto"/>
@@ -2148,10 +2148,10 @@
           <attributes>
             <attribute name="name" type="identifier"/>
             <attribute name="plugin" type="reference" reference_namespace="plugin"/>
-            <attribute name="instance" type="reference" reference_namespace="plugin_instance"/>
+            <attribute name="instance" type="reference" reference_namespace="plugin"/>
             <attribute name="cutoff" type="float"/>
-            <attribute name="objtype" type="keyword" required="true" valid_values="body joint geom site camera light mesh skin hfield texture material equality tendon actuator sensor numeric text tuple contact keyframe"/>
-            <attribute name="objname" type="reference" required="true" reference_namespace="attrib:objtype"/>
+            <attribute name="objtype" type="keyword" valid_values="body joint geom site camera light mesh skin hfield texture material equality tendon actuator sensor numeric text tuple contact keyframe"/>
+            <attribute name="objname" type="reference" reference_namespace="attrib:objtype"/>
             <attribute name="user" type="array" array_type="float"/>
           </attributes>
           <children>