reppoint bug fix when negative image training

[MinkiSong]

bug fix when negative image training using reppoint head

Motivation

error when number of gt bbox is zero

Modification

1. Added exception handling to get_pos_loss function of RotatedRepPointsHead class.
2. Added exception handling to convex_overlaps function of MaxConvexIoUAssigner class.

[codecov]

Codecov Report

Merging #396 (aedb8ae) into main (01439d1) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #396      +/-   ##
==========================================
- Coverage   30.85%   30.83%   -0.02%     
==========================================
  Files         119      119              
  Lines        7936     7941       +5     
  Branches     1221     1223       +2     
==========================================
  Hits         2449     2449              
- Misses       5386     5391       +5     
  Partials      101      101              

Flag	Coverage Δ
unittests	30.81% <0.00%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...ate/core/bbox/assigners/max_convex_iou_assigner.py	13.75% <0.00%> (-0.36%)	⬇️
...otate/models/dense_heads/rotated_reppoints_head.py	6.77% <0.00%> (-0.05%)	⬇️

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[zytx121]

LGTM

[zytx121]

Thanks for your PR. It's helpful for MMRotate.