Implement childPipeline properly

[hungyanbin]

Now the Pipeline class have a member variable called childPipeline, but it's not being used, We need to override BasePipelineSetupScope.pipeline() to when calling from parent Pipeline.

[hungyanbin]

Slack note:
但是如果child的話應該就要直接有parent的概念跟reference

2:10
所以
2:11
GraphPipelineSetupScope < BasePipelineSetupScope < GlEnvDslScope
所以應該要override BasePipelineSetupScope的pipeline method
2:11
這樣他就知道parent是誰了
New
2:11
這樣這個method直接可以把自己加成parent的child
2:11
DSL設計起來滿麻煩的ＸＤＤ
2:11
不過現在這樣我覺得還算好用
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