[Enhancement] LR triangular

[gengenkai]

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Motivation

Add a linear triangular learning rate strategy.

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMCls.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov]

Codecov Report

Merging #1001 (9d8709c) into master (934b549) will increase coverage by 0.03%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master    #1001      +/-   ##
==========================================
+ Coverage   64.56%   64.59%   +0.03%     
==========================================
  Files         152      152              
  Lines        9800     9806       +6     
  Branches     1780     1783       +3     
==========================================
+ Hits         6327     6334       +7     
+ Misses       3144     3143       -1     
  Partials      329      329              

Flag	Coverage Δ
unittests	64.59% <85.71%> (+0.03%)	⬆️

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

Impacted Files	Coverage Δ
mmcv/runner/hooks/lr_updater.py	69.42% <85.71%> (+0.27%)	⬆️
mmcv/runner/iter_based_runner.py	56.83% <0.00%> (+0.71%)	⬆️
mmcv/runner/hooks/logger/pavi.py	70.58% <0.00%> (+1.47%)	⬆️

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[zhouzaida]

please add unittest

[ZwwWayne]

@gengenkai need to resolve conflicts before merge.