Add passive collision mode for flexes.

PiperOrigin-RevId: 801754907
Change-Id: I4147af2ee2596519d42cb9a03cf8d5488f0bc3e8

model/flex/sphere_passive.xml

@@ -0,0 +1,36 @@
+<!-- Copyright 2024 DeepMind Technologies Limited
+
+     Licensed under the Apache License, Version 2.0 (the "License");
+     you may not use this file except in compliance with the License.
+     You may obtain a copy of the License at
+
+         http://www.apache.org/licenses/LICENSE-2.0
+
+     Unless required by applicable law or agreed to in writing, software
+     distributed under the License is distributed on an "AS IS" BASIS,
+     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+     See the License for the specific language governing permissions and
+     limitations under the License.
+-->
+
+<mujoco model="Full-flex sphere">
+  <include file="scene.xml"/>
+
+  <option solver="CG" tolerance="1e-6" timestep=".001" integrator="implicitfast"/>
+
+  <size memory="10M"/>
+
+  <visual>
+    <map stiffness="500"/>
+  </visual>
+
+  <worldbody>
+    <geom type="box" pos="1.5 0 0.25" size=".5 2 .25"/>
+    <geom type="box" pos="0 0 0.25" size="2 2 .05" euler="0 15 0"/>
+    <flexcomp type="ellipsoid" count="8 8 8" spacing=".07 .07 .07" pos="-.5 0 1" dim="3"
+              radius=".001" rgba="0 .7 .7 1" mass="5" name="slow">
+      <edge equality="true"/>
+      <contact selfcollide="none" internal="false" passive="true"/>
+    </flexcomp>
+  </worldbody>
+</mujoco>