[Feature] Resume from the latest checkpoint automatically.

[HIT-cwh]

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Motivation

Resume from the latest checkpoint automatically without specifying the path of the checkpoint.
Modified from mmdet pr#6727

Modification

Add a new argument auto-resume in tools/mmcls/train.py, tools/mmdet/train.py and tools/mmseg/train.py, and add a new function find_latest_checkpoint in mmrazor/utils/misc.py.

BC-breaking (Optional)

Does the modification introduce changes that break the backward compatibility of the downstream repositories?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here and update the documentation.

Checklist

Before PR:

• Pre-commit or other linting tools are used to fix the potential lint issues.
• Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
• The modification is covered by complete unit tests. If not, please add more unit tests to ensure the correctness.
• The documentation has been modified accordingly, like docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with those projects, like MMDet or MMSeg.
• CLA has been signed and all committers have signed the CLA in this PR.

[codecov]

Codecov Report

Merging #61 (9b0be7a) into master (366fd0f) will decrease coverage by 0.04%.
The diff coverage is 56.09%.

@@            Coverage Diff             @@
##           master      #61      +/-   ##
==========================================
- Coverage   58.50%   58.46%   -0.05%     
==========================================
  Files          86       87       +1     
  Lines        3114     3154      +40     
  Branches      570      581      +11     
==========================================
+ Hits         1822     1844      +22     
- Misses       1205     1223      +18     
  Partials       87       87              

Flag	Coverage Δ
unittests	58.43% <56.09%> (-0.05%)	⬇️

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Impacted Files	Coverage Δ
mmrazor/apis/mmcls/train.py	0.00% <0.00%> (ø)
mmrazor/apis/mmdet/train.py	0.00% <0.00%> (ø)
mmrazor/apis/mmseg/train.py	0.00% <0.00%> (ø)
mmrazor/utils/__init__.py	100.00% <100.00%> (ø)
mmrazor/utils/misc.py	100.00% <100.00%> (ø)

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