[Bug Report] reset() returns stale camera images when fabric is enabled - num_rerenders_on_reset re-renders the old transforms

Issue title

reset() returns stale camera images when fabric is enabled — num_rerenders_on_reset re-renders the old transforms

Body

Setup: Isaac Sim 6.0.0, manager-based env with a CameraCfg sensor, use_fabric=True (default), and a reset event that randomizes asset poses. Repro'd on the arena branch (55df2c3439, v3.0.0-beta); the relevant code is identical on main.

What happens

If a reset event moves assets (e.g. reset events calling write_root_pose_to_sim), the observations returned by reset() contain camera images of the scene from before the reset, while all state observations are post-reset. A policy's first inference each episode sees the wrong scene, and recorded datasets have a corrupt first frame.

Repro in one number: teleport two tabletop objects a few cm in a reset event, then compare the image returned by reset() with the image after one zero-action step():

mean |reset_image − step_image| ≈ 5.0    # expected ≈ 0; the objects visibly
                                         # sit at their OLD positions

Setting num_rerenders_on_reset (the documented remedy) changes nothing (diff 4.96 with 2 rerenders, 4.98 with 0).

Why

With fabric enabled, poses written through the PhysX tensor API only reach the renderer during a physics step. reset() never steps physics:

self._reset_idx(env_ids)              # events teleport assets (PhysX tensor API)
self.scene.write_data_to_sim()
self.sim.forward()                    # does NOT sync PhysX -> fabric
for _ in range(self.cfg.num_rerenders_on_reset):
    self.sim.render()                 # re-renders the OLD fabric transforms
self.obs_buf = self.observation_manager.compute()   # image: old, state: new

Confirmed: ten consecutive sim.render() calls after such a reset still draw the old scene; one sim.step() (or use_fabric=False) makes the reset image correct (diff ≈ 0.3).

Suggested fix

When RTX sensors are present, sync the PhysX→fabric transforms (what a physics step does) before the num_rerenders_on_reset render loop in ManagerBasedEnv.reset() / the DirectRLEnv equivalent — or document that under fabric these images cannot be refreshed without stepping physics.