Single step the viewer render?

Is there a recommend way to single step the viewer renderer?

In the example code snippet, I would like to grab input (ex: keyboard presses for direction) and update viewer frames. Currently, viewer.launch(env) blocks.

```
if __name__ == "__main__":

physics = mujoco.Physics.from_xml_path(MODEL_PATH_XML)
task = Xarm7()
env = control.Environment(
physics, task, time_limit=_TIME_LIMIT, control_timestep=_CONTROL_TIMESTEP
)

action_spec = env.action_spec()
time_step = env.reset()
viewer.launch(env)
```
An alternative is to encode the teleoperation input as a policy, however it's a bit unclear how that structure would look. Any example snippets would greatly appreciated!

从示例入口点 `viewer.launch(env)` 以及 issue 中展示的周边环境设置开始。检查 viewer 当前如何阻塞，以及现有的 viewer 或环境示例是否提供 stepping 或 policy 回调。完成标准是：有一种经过文档说明且可复现的方式，在不出现阻塞行为的情况下提供输入并更新 viewer 帧。

由索引模型根据 Issue 内容生成。