Support Rotated FCOS

[zytx121]

Description

This issue describes the development requirement of a feature we wish to collaborate with the community.
In this issue, we welcome the community developers to participate in the development of FCOS.

To inherit a new FCOSHead in MMRotate to support OBBs.

Steps

• Implement the RotatedFCOSHead
• Benchmark the performance
• Prepare and release the models

Resources

• https://github.com/open-mmlab/mmdetection/blob/master/mmdet/models/dense_heads/fcos_head.py
• https://github.com/jbwang1997/OBBDetection/blob/master/mmdet/models/dense_heads/obb/obb_fcos_head.py