Mark "count" attribute on "composite" as non-required.

For cable composites, either vertex or count should be specified.

Fixes #439.

PiperOrigin-RevId: 592525932
Change-Id: I3da34784547df8a8c6064b47b6b8f03d5ce3b9b0

Author: nimrod-gileadi

dm_control/mjcf/schema.xml

@@ -1157,7 +1157,7 @@
           <attributes>
             <attribute name="prefix" type="identifier"/>
             <attribute name="type" type="keyword" valid_values="particle grid cable rope loop cloth box cylinder ellipsoid" required="true"/>
-            <attribute name="count" type="array" array_type="int" array_size="3" required="true"/>
+            <attribute name="count" type="array" array_type="int" array_size="3"/>
             <attribute name="spacing" type="float" required="false"/>
             <attribute name="offset" type="array" array_type="float" array_size="3"/>
             <attribute name="flatinertia" type="float"/>
@@ -1542,7 +1542,7 @@
               <attributes>
                 <attribute name="prefix" type="identifier"/>
                 <attribute name="type" type="keyword" valid_values="particle grid cable rope loop cloth box cylinder ellipsoid" required="true"/>
-                <attribute name="count" type="array" array_type="int" array_size="3" required="true"/>
+                <attribute name="count" type="array" array_type="int" array_size="3"/>
                 <attribute name="spacing" type="float" required="false"/>
                 <attribute name="offset" type="array" array_type="float" array_size="3"/>
                 <attribute name="flatinertia" type="float"/>