Node - #activity#return

For Xmind ChatTree, the first line of the node content must be "#activity#return", and then there is no need for other lines of content, such as:
```
#activity#return
```
For Python ChatTree, the first parameter of chattree.create_node() must be "#activity#return", and the second parameter dict is empty, such as:
```
xxx_node = chattree.create_node("#activity#return", {})
```
Immediately return from the called sub-ChatTree or triggered sub-dialogue-process to the node that initiated the sub-tree call or the node before the trigger. When there is no sub-tree or sub-dialogue-process to return to, end the entire dialogue.