[Fix] Use MMCV's EvalHook in MMClassification

[LXXXXR]

Hi,
This pull request uses EvalHook from mmcv(open-mmlab/mmcv#739) to replace ones in mmcls. Both are supported for mmcls v0.10.0, but the after that, EvalHook and DistEvalHook will be removed.

Pls kindly take a look. Thx.

[codecov]

Codecov Report

Merging #182 (a9356ca) into master (49ece04) will increase coverage by 0.05%.
The diff coverage is 90.00%.

@@            Coverage Diff             @@
##           master     #182      +/-   ##
==========================================
+ Coverage   71.88%   71.94%   +0.05%     
==========================================
  Files          82       82              
  Lines        3802     3810       +8     
  Branches      687      687              
==========================================
+ Hits         2733     2741       +8     
  Misses        987      987              
  Partials       82       82              

Flag	Coverage Δ
unittests	71.94% <90.00%> (+0.05%)	⬆️

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

Impacted Files	Coverage Δ
mmcls/apis/train.py	29.03% <85.71%> (+6.22%)	⬆️
mmcls/core/evaluation/eval_hooks.py	89.65% <100.00%> (+0.56%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 49ece04...a9356ca. Read the comment docs.

[ycxioooong]

LGTM