Explicitly specify actdim on PID plugin actuators, to allow them to be used with keyframes.

Support actdim attribute on plugin actuators.

Remove the actuator_actdim callback. This callback leads to crashes when loading keyframes for models that have a stateful plugin.

PiperOrigin-RevId: 650649505
Change-Id: I0171a75927a895b5f1a19f69244a17f2da801bcd

Author: nimrod-gileadi

dm_control/mjcf/schema.xml

@@ -2167,6 +2167,7 @@
             <attribute name="forcerange" type="array" array_type="float" array_size="2"/>
             <attribute name="actrange" type="array" array_type="float" array_size="2"/>
             <attribute name="lengthrange" type="array" array_type="float" array_size="2"/>
+            <attribute name="actdim" type="int"/>
             <attribute name="dyntype" type="keyword" valid_values="none integrator filter filterexact muscle user"/>
             <attribute name="dynprm" type="array" array_type="float" array_size="10"/>
             <attribute name="gear" type="array" array_type="float" array_size="6"/>