[streaming] tree pipeline support

[annasun28]

• supports multiple leaf outputs, and passing output from an intermediate module to multiple downstream modules
• Use the option --output-index in TreeAgentPipeline to output only one of the multiple outputs (used for testing w/existing dataloaders)

[xutaima]

@annasun28 Thank you so much!!!! This is really cool! Will take a deeper look later this week.