[Fix] Fix train error in EpochBasedRunner

[zhouzaida]

Related PR #925

[codecov]

Codecov Report

Merging #942 (b9f19aa) into master (79f8cbd) will not change coverage.
The diff coverage is 100.00%.

❗ Current head b9f19aa differs from pull request most recent head 6685993. Consider uploading reports for the commit 6685993 to get more accurate results

@@           Coverage Diff           @@
##           master     #942   +/-   ##
=======================================
  Coverage   65.67%   65.67%           
=======================================
  Files         149      149           
  Lines        9451     9451           
  Branches     1720     1720           
=======================================
  Hits         6207     6207           
  Misses       2924     2924           
  Partials      320      320           

Flag	Coverage Δ
unittests	65.67% <100.00%> (ø)

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Impacted Files	Coverage Δ
mmcv/runner/epoch_based_runner.py	82.69% <100.00%> (ø)

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