[Feature] Support Res2Net

[Junjun2016]

Motivation

Support more backbones for semantic segmentation. In this PR, we add Res2Net.

Modification

• [x] Add Res2Net backbone and unitests
• [x] Add configs for Res2Net
• [ ] Add benchmarks for Res2Net

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repos? No

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 90.52632% with 9 lines in your changes missing coverage. Please review.

Project coverage is 85.36%. Comparing base (4d34581) to head (aadfd02). Report is 387 commits behind head on master.

Files	Patch %	Lines
mmseg/models/backbones/res2net.py	90.21%	4 Missing and 5 partials :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #722      +/-   ##
==========================================
+ Coverage   85.28%   85.36%   +0.08%     
==========================================
  Files         107      108       +1     
  Lines        5817     5912      +95     
  Branches      952      968      +16     
==========================================
+ Hits         4961     5047      +86     
- Misses        673      677       +4     
- Partials      183      188       +5     

Flag	Coverage Δ
unittests	85.35% <90.52%> (+0.08%)	:arrow_up:

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

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.