Use nstep argument for physics.step in Cheetah initialization.

This was suggested by Trinkle23897 on bd6d893.

PiperOrigin-RevId: 448234630
Change-Id: Id6dacbbb2a9e7779b7a5e616d6365b390c0099ab

Author: nimrod-gileadi

dm_control/suite/cheetah.py

@@ -69,8 +69,7 @@ def initialize_episode(self, physics):
     physics.data.qpos[is_limited] = self.random.uniform(lower, upper)
 
     # Stabilize the model before the actual simulation.
-    for _ in range(200):
-      physics.step()
+    physics.step(nstep=200)
 
     physics.data.time = 0
     self._timeout_progress = 0