Optionally enable actuation in `physics.after_reset`?

Would it make sense to optionally enable actuation in the `after_reset` method [here](https://github.com/google-deepmind/dm_control/blob/main/dm_control/mujoco/engine.py#L310)?

The use case I have in mind is initializing episode with actuators in a certain activation state `physics.data.act`, before any control input is provided. What do you think?

首先阅读链接位置的 dm_control/mujoco/engine.py 中的 after_reset 方法，以及周围的 reset 行为。确定如何在保留当前默认行为的同时，让可选的执行路径在控制输入之前初始化 physics.data.act；当该行为被定义并一致地实现后，issue 即完成。

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