[Enhance] Unify the interface of stuff head and panoptic head

[AronLin]

Replace fg_range and bg_range with num_thing_classes and num_stuff_classes.
Deprecated two interfaces fg_range and bg_range.

[codecov]

Codecov Report

Merging #6308 (023fe4c) into master (a32d7d4) will increase coverage by 0.02%.
The diff coverage is 5.88%.

@@            Coverage Diff             @@
##           master    #6308      +/-   ##
==========================================
+ Coverage   61.82%   61.85%   +0.02%     
==========================================
  Files         315      315              
  Lines       25166    25174       +8     
  Branches     4185     4187       +2     
==========================================
+ Hits        15560    15572      +12     
+ Misses       8797     8789       -8     
- Partials      809      813       +4     

Flag	Coverage Δ
unittests	61.83% <5.88%> (+0.02%)	⬆️

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

Impacted Files	Coverage Δ
mmdet/models/seg_heads/panoptic_fpn_head.py	25.45% <5.88%> (-2.21%)	⬇️
...et/models/roi_heads/bbox_heads/convfc_bbox_head.py	55.88% <0.00%> (ø)
mmdet/models/dense_heads/corner_head.py	69.67% <0.00%> (+2.45%)	⬆️
mmdet/models/detectors/cornernet.py	100.00% <0.00%> (+5.12%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a32d7d4...023fe4c. Read the comment docs.

[AronLin]

I will train a model to ensure that the modification is correct.