Make sure that contact_force always returns the correct values.

The `physics.step` function in `dm_control` runs `mj_step2` followed by `mj_step1`. This leaves the physics in a state where `mj_fwdPosition` has been called, and therefore the contact frames have been updated, but`mj_fwdConstraint` has not, and therefore the contact forces are stale. This change forces the portion of `mj_step2` that is needed to guarantee correct contact forces to be always be run as part of the `contact_force` function.

Fixes google-deepmind/mujoco#13.

PiperOrigin-RevId: 490922782
Change-Id: I0a3998ff4759aee70321a636c189e44de2f3eca5

Author: saran-t

dm_control/mujoco/wrapper/core.py

@@ -521,6 +521,12 @@ def contact_force(self, contact_id):
     if not 0 <= contact_id < self.ncon:
       raise ValueError(_CONTACT_ID_OUT_OF_RANGE
                        .format(max_valid=self.ncon-1, actual=contact_id))
+
+    # Run the portion of `mj_step2` that are needed for correct contact forces.
+    mujoco.mj_fwdActuation(self._model.ptr, self._data)
+    mujoco.mj_fwdAcceleration(self._model.ptr, self._data)
+    mujoco.mj_fwdConstraint(self._model.ptr, self._data)
+
     wrench = np.empty(6, dtype=np.float64)
     mujoco.mj_contactForce(self._model.ptr, self._data, contact_id, wrench)
     return wrench.reshape(2, 3)