[Feature] Support Tversky Loss

[MengzhangLI]

Motivation

Adding new loss function named Tversky Loss. https://arxiv.org/abs/1706.05721

Old PR: #634, #919

Modification

Datasets	loss_ce (from repo config)	loss_ce + loss_tversky (1:1)	loss_ce + loss_tversky (1:3)	loss_tversky
DRIVE	78.67	79.39	79.81	79.72
STARE	81.02	82.3	82.62	82.03
CHASE_DB1	80.24	80.37	79.74	77.95
HRF	79.45	81.03	80.81	--

[codecov]

Codecov Report

Merging #1986 (c2c7d00) into master (13d4c39) will increase coverage by 0.03%.
The diff coverage is 94.64%.

@@            Coverage Diff             @@
##           master    #1986      +/-   ##
==========================================
+ Coverage   89.04%   89.08%   +0.03%     
==========================================
  Files         144      145       +1     
  Lines        8636     8691      +55     
  Branches     1458     1463       +5     
==========================================
+ Hits         7690     7742      +52     
- Misses        706      707       +1     
- Partials      240      242       +2     

Flag	Coverage Δ
unittests	89.08% <94.64%> (+0.03%)	⬆️

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

Impacted Files	Coverage Δ
mmseg/models/losses/tversky_loss.py	94.44% <94.44%> (ø)
mmseg/models/losses/__init__.py	100.00% <100.00%> (ø)
mmseg/version.py	58.33% <100.00%> (ø)

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