Keep train/valand eval configs separated

[mawanda-jun]

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When structuring the configuration, it's useful to keep the training, validation and evaluation configurations separated. In this case, it'd be useful to have a test configuration all alone, letting a separated validation configuration.

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Codecov Report

Merging #1760 (9bdf791) into master (8556944) will not change coverage. The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master    #1760   +/-   ##
=======================================
  Coverage   89.04%   89.04%           
=======================================
  Files         144      144           
  Lines        8636     8636           
  Branches     1458     1458           
=======================================
  Hits         7690     7690           
  Misses        706      706           
  Partials      240      240           

Flag	Coverage Δ
unittests	89.04% <0.00%> (ø)

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Impacted Files	Coverage Δ
mmseg/apis/train.py	19.51% <0.00%> (ø)

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