[RELAY][FRONEND] Frontend scalfolding

[tqchen]

This is an issue to coordinate frontend scaffolding for relay. To help us coordinate, I have tagged a list of folks who originated the initial scaffolding of nnvm part to initiate the move, please feel free to reply to this issue and coordinate the tasks.

• ONNX @zhreshold
• Keras @Huyuwei
• Tensorflow @srkreddy1238
• DarkNet @PariksheetPinjari909
• Caffe2
• CoreML
• NNVMv1 to relay
• MXNet

[srkreddy1238]

#2216 for tensorflow

[zhreshold]

#2302 for onnx

[Huyuwei]

#2336 for keras

[tqchen]

@Huyuwei can you also help to migrate the frontend tutorial into tutorials/frontend?

[Huyuwei]

#2479 migrates from_keras tutorial to tutorials/frontend

[makihiro]

#2507 for Caffe2

[tqchen]

We still have CoreML left, @kazum would you be interested in help on that?

[zhiics]

Is #2476 the CoreML frontend?