Add mask target generator operator for Mask-RCNN

[Kh4L]

Description

Add mrcnn_target GPU operator.

It computes Mask-RCNN mask targets. It takes the same arguments and produces the same outputs as this Gluon block.

Micro-benchmark:

mrcnn_target operator 374ns vs Reference 5.43μs

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage:
• Unit tests are added for small changes to verify correctness (e.g. adding a new operator)
• Code is well-documented:
• For user-facing API changes, API doc string has been updated.
• For new C++ functions in header files, their functionalities and arguments are documented.
• For new examples, README.md is added to explain the what the example does, the source of the dataset, expected performance on test set and reference to the original paper if applicable
• To the my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

Changes

• Operator, tests, (and when applicable, API doc)

Comments

• Add type support
• Need to test Symbol API

[anirudhacharya]

Micro-benchmark:

mrcnn_target operator 374ns vs Reference 5.43μs

What is the reference in this benchmark?

[Jerryzcn]

Micro-benchmark:

mrcnn_target operator 374ns vs Reference 5.43μs

What is the reference in this benchmark?

Probably the current python version

[Kh4L]

mrcnn_target operator 374ns vs Reference 5.43μs

@anirudhacharya

@Jerryzcn is right, it is this Gluon code, hybridized.

[wkcn]

LGTM. Thank you!