Modify 'val_step()' to validate data for each val mode epoch

[dreamerlin]

An alternative validation process pointed out by #115, but we still suggest using --validate to call EvalHook and DistEvalHook for evaluation.
Related modification in mmcv is also submitted

[codecov]

Codecov Report

Merging #123 into master will decrease coverage by 0.01%.
The diff coverage is 16.66%.

@@            Coverage Diff             @@
##           master     #123      +/-   ##
==========================================
- Coverage   85.53%   85.51%   -0.02%     
==========================================
  Files          76       76              
  Lines        4155     4157       +2     
  Branches      662      662              
==========================================
+ Hits         3554     3555       +1     
- Misses        494      496       +2     
+ Partials      107      106       -1     

Flag	Coverage Δ
#unittests	85.51% <16.66%> (-0.02%)	⬇️

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Impacted Files	Coverage Δ
mmaction/models/backbones/resnet_tsm.py	95.08% <ø> (ø)
mmaction/models/recognizers/base.py	59.72% <0.00%> (-1.71%)	⬇️
mmaction/models/backbones/resnet3d_csn.py	100.00% <100.00%> (+2.63%)	⬆️

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