[RFC] add "Unique", "DenseToDenseSetOperation" and "SparseToDense" relay op

Development pre-RFC
#1

[Motivation] use Relay to support the TensorFlow MaskRCNN model.

[recovery the errors]

  1. down load the MaskRCNN model from “https://github.com/LakeFeiLiu/Mask_RCNN”
  2. python the path/to/sample/maskrcnn.py

[errors]

“NotImplementedError: The following operators are not implemented: {‘SparseToDense’, ‘DenseToDenseSetOperation’, ‘Unique’}”

[TODO]

to support the three operators ‘SparseToDense’, ‘DenseToDenseSetOperation’, ‘Unique’ in Relay and run passed on CPU.

if anyone do the same work, please contract me!

thanks !