[Fix] Fix the bug that the training log and evaluating log are mixed

[zhouzaida]

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Motivation

Fix the bug that the training log and evaluating log are mixed.

Because the priority of EvalHook is higher than LoggerHook, the training log and the evaluating log are mixed. Therefore, we need to dump the training log and clear it before evaluating log is generated. In addition, this problem will only appear in IterBasedRunner whose self.by_epoch is False, because EpochBasedRunner whose self.by_epoch is True calls _do_evaluate in after_train_epoch stage, and at this stage the training log has been printed, so it will not cause any problem.

more details

• Log for val results does not save correctly when IterBasedRunner is used. #1248
• Evaluation results logging is inconsistent #1058
• False output of 'eval' result in the training log mmsegmentation#694

Modification

Add some lines in mmcv/runner/hooks/evaluation.py

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repos?
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

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 #1252 (2f934a8) into master (04daea4) will increase coverage by 0.13%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1252      +/-   ##
==========================================
+ Coverage   68.27%   68.40%   +0.13%     
==========================================
  Files         160      160              
  Lines       10599    10608       +9     
  Branches     1937     1938       +1     
==========================================
+ Hits         7236     7256      +20     
+ Misses       2979     2976       -3     
+ Partials      384      376       -8     

Flag	Coverage Δ
unittests	68.40% <100.00%> (+0.13%)	⬆️

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

Impacted Files	Coverage Δ
mmcv/runner/hooks/evaluation.py	89.08% <100.00%> (+2.37%)	⬆️
mmcv/utils/parrots_wrapper.py	47.76% <0.00%> (-11.62%)	⬇️
mmcv/utils/env.py	56.36% <0.00%> (-0.25%)	⬇️
mmcv/version.py	82.35% <0.00%> (ø)
mmcv/ops/bbox.py	15.00% <0.00%> (ø)
mmcv/ops/info.py	61.90% <0.00%> (ø)
mmcv/onnx/info.py	68.75% <0.00%> (ø)
mmcv/image/misc.py	80.95% <0.00%> (ø)
mmcv/ops/carafe.py	23.30% <0.00%> (ø)
mmcv/ops/saconv.py	83.33% <0.00%> (ø)
... and 70 more

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