[Feature] Add feature refine CUDA op for rotated detection.

[zytx121]

Motivation

Add feature refine CUDA operation for rotate detection.

@inproceedings{yang2021r3det,
    title={R3Det: Refined Single-Stage Detector with Feature Refinement for Rotating Object},
    author={Yang, Xue and Yan, Junchi and Feng, Ziming and He, Tao},
    booktitle={Proceedings of the AAAI Conference on Artificial Intelligence},
    volume={35},
    number={4},
    pages={3163--3171},
    year={2021}
}

Modification

Several files in mmcv/ops folder.

BC-breaking (Optional)

No.

Use cases (Optional)

from mmcv.ops import feature_refine

[zhouzaida]

Unit test failed in my local machine.

More details can be found at pytorch/pytorch#16940.

[zytx121]

Unit test failed in my local machine. More details can be found at pytorch/pytorch#16940.

Thanks for your web link. We have fixed it.