support streaming output of sub-workflows

[hieheihei]

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1. Is this request related to a challenge you're experiencing? Tell me about your story.

I'm trying to call the chat-workflow API to get a streaming response, but the response from the sub-workflow node is blocking.

As shown in the figure, there is a main workflow and a tool sub-workflow.

• node 1:streaming
• node 2:blocking(ok)
• node 3:blocking(sub-workflow,need streaming)
• node 4:streaming

the sub-workflow node 3 runs in a streaming manner, but when the main workflow is called, it has to wait until node 3 finishes running completely before there is an output all at once.

2. Additional context or comments

No response

3. Can you help us with this feature?

• [ ] I am interested in contributing to this feature.