Stream-based Pipeline & Plugins

[WenheLI]

We have implemented a steam-based pipeline for tensorflow-image classification, and we got a huge performance boost in terms of execution time (see #410 )

We should also modify the rest of plugins here:

• pipcook-plugin-tensorflow-bert-ner-model-train

- [ ] [pipcook-plugin-tensorflow-image-generation-model-train](https://github.com/imgcook/pipcook-plugin-tensorflow-image-generation-model-train) (use batch)

[ZijinGao]

Hi! I'm interested in contributing to the pipcook-plugin-tensorflow-bert-ner-model-train plugin. Would like to know it in details.

[FeelyChau]

In v2, all the scripts are stream-based. This issue is not applicable anymore. Closed.