Add implicitfast integrator to PyMJCF schema.

PiperOrigin-RevId: 517138279
Change-Id: Ie40dd749c99a60971c99930ff612a78b81727654

Author: yuvaltassa

dm_control/mjcf/schema.xml

@@ -56,7 +56,7 @@
         <attribute name="o_margin" type="float"/>
         <attribute name="o_solref" type="array" array_type="float" array_size="2"/>
         <attribute name="o_solimp" type="array" array_type="float" array_size="5"/>
-        <attribute name="integrator" type="keyword" valid_values="Euler RK4 implicit"/>
+        <attribute name="integrator" type="keyword" valid_values="Euler RK4 implicit implicitfast"/>
         <attribute name="collision" type="keyword" valid_values="all predefined dynamic"/>
         <attribute name="cone" type="keyword" valid_values="pyramidal elliptic"/>
         <attribute name="jacobian" type="keyword" valid_values="dense sparse auto"/>