[Features] Support auto-resume

[MeowZheng]

1. Add find_latest_checkpoint which will be used when auto-resume training
2. Add argument auto-resume in tools/train.py

Related PR: https://github.com/open-mmlab/mmdetection/pull/6727/files

[codecov]

Codecov Report

Merging #71 (136fc14) into master (aeebb1e) will increase coverage by 0.22%.
The diff coverage is 82.75%.

@@            Coverage Diff             @@
##           master      #71      +/-   ##
==========================================
+ Coverage   67.72%   67.95%   +0.22%     
==========================================
  Files          84       85       +1     
  Lines        4828     4855      +27     
  Branches      689      696       +7     
==========================================
+ Hits         3270     3299      +29     
+ Misses       1454     1452       -2     
  Partials      104      104              

Flag	Coverage Δ
unittests	67.95% <82.75%> (+0.22%)	⬆️

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Impacted Files	Coverage Δ
mmflow/apis/train.py	19.54% <16.66%> (-1.20%)	⬇️
mmflow/utils/__init__.py	100.00% <100.00%> (ø)
mmflow/utils/misc.py	100.00% <100.00%> (ø)
mmflow/datasets/pipelines/transforms.py	79.83% <0.00%> (+1.16%)	⬆️

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